Abstracts

Below you can find the abstracts of the different talks to be presented during the IAUS321:

Title of oral contributionAbstractName (First)Name (Last)AffiliationType
Unresolved stellar halos and streamsI will review recent progress in measuring the properties of stellar halos and streams using deep imaging observations that do not resolve structures into individual stars. I will focus on the following questions: What surface brightness limits are "interesting" according to the predictions of models? Do current observations hit these limits? If not, why not? And, finally, how might one harness new technologies and new techniques in order reach the sorts of "ultra deep" limits currently being probed by studies of resolved stellar populations in the Local Group?RobertoAbrahamDunlap Institute for Astronomy & AstrophysicsReview
Outskirts of spiral galaxiesThis presentation reviews the outer regions of nearby spiral galaxies, with emphasis on their star formation and chemical abundance properties.FabioBresolinUniversity of HawaiiReview
Cosmological-context models of galaxy outskirtsI will discuss the key aspects that galaxy outskirts play in the role of galaxy evolution, particularly in relationship to the baryon cycle. I will present a new suite of simulations using Gizmo's meshless hydrodynamics with state of the art feedback prescriptions. I will emphasize the key roles played by metallicity and neutral hydrogen at the interface of galaxies and the circumgalactic medium, and highlight ways in which this interface can be probed through coordinated programs in emission and absorption.RomeelDav‚éUniversity of Western CapeReview
Outer regions and growth of high-redshift disk galaxiesI will discuss recent key progress in our knowledge about the growth of disk galaxies at z ~ 1 - 3, with an emphasis on spatially-resolved studies of their stellar and gas content, and of their gas kinematics. I will highlight challenges and successes in probing the outer regions of high-z galaxies, and will outline implications for our understanding of physical processes driving galaxy evolution from mass assembly to feedback.Natascha M.Forster SchreiberMax-Planck-Institut fuer extraterrestrische PhysikReview
The Outskirts of Early-Type GalaxiesIn this talk I will review a number of recent works studying the buildup of the outer parts of early-type galaxies. Most of the results are based on deep wide-area imaging surveys, such as the NGVS survey and the Fornax Deep Survey FDS. Questions to discuss are what the role of the environment is in shaping the outer regions, and what the star formation histories are.ReynierPeletierKapteyn Institute, University of GroningenReview
The MUSE Atlas of DisksMost of stellar growth in galaxies over the past ten or more billion years takes place on the 'Star Forming Main Sequence' in which the star-formation rate is proportional to the existing galaxy stellar mass. The MUSE Atlas of Disks (MAD) program of 2-D integral-field spectroscopy on the VLT for a large representative sample of z=0 Main Sequence galaxies simultaneously achieves much higher spatial resolution, denser sampling and greater depth than any other ongoing program of 2-D spectroscopy on such systems. By mapping stellar and gaseous kinematics and absorption/emission line properties (stellar populations, metallicity, dust extinction, SFRs, ISM properties) at about 100pc resolution out to up to two disk scale lengths, MAD aims at establishing the dynamical state of inner disks and bulges and their star-formation and assembly histories, detect gas in/outflows, shocks, kinematic and chemical/population signatures of accretion events, and reveal connections between baryonic and dark matter halo properties. This is key towards clarifying how stars and gas interact within galaxies on the small scales over which stars form and return energy feedback, what conditions halt or stimulate star formation, how heavy elements are redistributed across galactic bodies, and whether gas outflows play a significant role in keeping galaxies on the Main Sequence equilibrium -- and thus, ultimately, what governs the relative mass and size growth with cosmic time of bulges and outer disks in the active phase of galactic life on the Main Sequence. In this talk I will focus specifically on the question whether galaxies with profoundly different outer-disk properties (down- and up-bending surface brightness profiles) show also distinctive properties in their bulge and inner-disk components, and present results from our first-epoch MAD data which provide evidence that the answer is: yes!MarcellaCarolloETH ZrichInvited
DLAs and outskirts of galaxiesUsing the largest homogenous survey of z > 4.4 damped Lyman alpha systems (DLAs), our team has made the most precise high-redshift measurement of the cosmological mass density of neutral hydrogen, HI. At such high redshift important systematic uncertainties in DLA identification are produced by strong intergalactic medium absorption. We correct for these effects using a combination of mock and a high-resolution spectra, and show that for our sample these uncertainties are smaller than the statistical errors. By comparing to measurements at lower redshift and considering systematic effects that might affect previous results, we show that the HI mass density shows little evolution from z=5 to 0. I will discuss the implications for how HI gas fuels star formation, and for how the intergalactic medium drives galaxy formation.NeilCrightonCentre for Astrophysics and Supercomputing, Swinburne UniversityInvited
Outskirts of the Milky WayThe high quality astrometric and photometric data already collected by Gaia during these almost two years of successful scientific operation allow us to anticipate that, in few years, Gaia will revolutionize our understanding of the Milky Way. In this talk I will shortly describe the mission status and the contents of the first and end-of-mission Data Releases (end of summer 2016 and 2022). More in detail, we will quantify the expect accuracy in position, velocity and astrophysical parameters of some key tracers of the stellar populations in the outskirts of the Milky Way.

It's time to address the major challenge of deriving the Milky Way disk Star Formation History and evolution from both the Gaia and the oncoming spectroscopic on-ground data (APOGEE, EMIR, WEAVE ...). The theoretical and modelling work being undertaken inside our team will be presented. This work aims to disclose the mechanisms driving the bar, the spiral arms and the warp formation and evolution though local and global measures of the 6D phase space distribution function which will be used to encode valuable dynamical information.
FrancescaFiguerasUniversity of Barcelona (IEEC-UB)Invited
Outer Disk Star Formation in HI Rich GalaxiesIt has long been known that the HI distribution in galaxies often extends past the optical extent of galaxies, as seen in conventional images. These outer disks are not dormant but dimly forming stars. I report results from our team which has been probing low intensity star formation in outer disks and dwarf galaxies. We have been using a variety of techniques including imaging in H-alpha and ultraviolet, from the ground and space, as well as high resolution imaging with the Hubble Space Telescope (HST). Analysis of the main sequence luminosity function of resolved stellar populations in two galaxies (as observed by HST) indicates that low intensity star formation is deficient in high-mass ionizing stars, in agreement with previous H-alpha and UV studies. Using a larger sample of HI selected galaxies, spanning about 5 dex in stellar mass, we confirm that outer disk HII regions and extended UV disks are common. We show that although the ultraviolet light in galaxies is more centrally concentrated than the HI, that the UV/HI ratio (equivalent to the Star Formation Efficiency) is nearly constant in HI selected galaxies, with a slight dependency on optical surface brightness. This result can be understood in the context of the disks in galaxies evolving towards having a nearly constant stability parameter Q.GerhardtMeurerUniversity of Western AustraliaInvited
Galaxy outskirts in the EAGLE simulationsI will report recent results related to galaxy outskirts from the EAGLE simulation project.JoopSchayeLeiden ObservatoryInvited
Ultra-deep imaging of nearby galaxy outskirts from the groundThe detection of optical surface brightness structures in the sky with magnitudes fainter than 30 mag/arcsec^2 has remained elusive in current photometric deep surveys. Here we show how present-day 10 meter class telescopes can provide broadband imaging 1.5-2 mag deeper than most previous results within a reasonable amount of time (i.e. <10h on source integration). In particular, we illustrate the ability of the 10.4 m Gran Telescopio de Canarias (GTC) telescope to produce imaging with a limiting surface brightness of 31.5 mag/arcsec^2. We apply this power to explore the stellar halos of nearby galaxies obtaining surface brightness radial profiles down to mu_r~33 mag/arcsec^2. This depth is similar to that obtained using star counts techniques of Local Group galaxies, but is achieved at a distance where this technique is unfeasible. IgnacioTrujilloInstituto de Astrofisica de CanariasInvited
Tracing high-z galaxy kinematics from turbulent disks to quenched spheroidsKinematics and structural properties have revealed that the majority of 'normal' star-forming galaxies at z~1-3 host disk-like structure. Using recent observations from large near-infrared galaxy surveys, I will discuss the formation and evolutionary paths of high redshift disks. In particular, I will present results from the KMOS^3D survey, an integral field survey of over 600 galaxies at z=0.7-2.7 using KMOS at the VLT. We compare the gas velocity dispersion in the outer regions of massive galaxies with data spanning from z=4 to z=0. The un-biased selection of KMOS^3D allows us to explore galaxy dynamics both on and below the star-forming 'main sequence' opening up new avenues in investigating evolutionary links between turbulent disks and quenched spheroids.EmilyWisnioskiMPEInvited
Exploring the stellar halo profile - accretion history connection on statistical samples of mock haloes of Lstar galaxiesThe assembly histories of galaxies and their dark matter haloes are directly mirrored in the properties of their accreted stellar haloes. Each accretion event deposits stars in dynamical locations that depend primarily on the satellite's total mass at infall and on its accretion time. Hence, the density and kinematical profiles of the accreted stellar halo are a unique window onto a galaxy's assembly history. This is especially true for Lstar galaxies, as their accreted haloes are mostly assembled by just a handful of main contributions, and are not easily buried by the plethora of only minor additions.Nicola CristianoAmoriscoMPA Garching & ITC HarvardOral contribution
Accretion phenomena in nearby starburst dwarf galaxiesStudies of the most actively star-forming dwarf galaxies in the local Universe are of paramount importance for Cosmology, both to better understand the conspicuous population of extreme starburst dwarfs recently discovered at z~1-2 in deep near-infrared surveys, and to test our ideas about the starburst phenomenon, which was more common in the early Universe due to the larger amount of gas and to the higher frequency of interactions and mergers.

Recent HI studies, and deep wide-field optical and near infrared imaging studies, provide increasing evidence that strong starbursts in local dwarf galaxies may have been triggered by accretion/merging events. In order to more deeply investigate this scenario, we are undergoing a project aimed at studying, through wide-field deep imaging and spectroscopy, signatures of accretion/interaction in a sample of very actively star forming local dwarfs previously studied by our group with HST. The wide field observations, coupled with the star formation histories already derived from HST photometry of the resolved stars, will be analyzed by means of N-Body simulations to simultaneously constrain the dynamical and SF timescales. I will present our results for two very actively star forming dwarf galaxies, NGC 4449 and DDO 68.
FrancescaAnnibaliINAF-OABO / DIFAOral contribution
Stellar populations in the outskirts of M31: the mid-infrared viewThe mid-infrared provides a unique view of galaxy stellar populations, sensitive to both the integrated light of old, low-mass stars and to individual dusty mass-losing stars. We present results from an extended Spitzer/IRAC survey of M31 with total lengths of 6.6 and 4.4 degrees along the major and minor axes, respectively. The integrated surface brightness profile proves to be surprisingly difficult to trace in the ouskirts of the galaxy, but we can also investigate the disk/halo transition via a star count profile, with careful correction for foreground and background contamination. Our point-source catalog allows us to report on mid-infrared properties of individual objects in the outskirts of M31, via cross-correlation with PAndAS, WISE, and other catalogs.PaulineBarmbyWestern University, CanadaOral contribution
A wide-area chemo-dynamical survey of individual stars in a transforming dwarf galaxyThe evolutionary link between late- and early-type dwarf galaxies is not yet understood, nor is the interplay of internal versus external mechanisms in determining their observed properties. Yet, this has fundamental implications for our understanding of galaxy formation and evolution at the low end of the galaxy mass function. In this contribution I will discuss the leap forward that studies of large numbers of individual stars over a wide-area have brought in the observational picture of the chemo-dynamical properties of Local Group dwarf galaxies. I will also present recent results from our VLT/FORS2 chemo-dynamical survey of ~200 individual member stars in Phoenix, the closest late-type dwarf galaxy arguably caught in the process of transforming into an early-type system.GiuseppinaBattagliaInstituto de Astrofisica de Canarias, TenerifeOral contribution
Rethinking the Narrative of a Micro-Merger: Tidal Debris with On-Going Star FormationIn the Lambda-CDM paradigm, Milky Way sized galaxies are assembled via many minor- (1:5 to 1:10 mass ratio) and micro-mergers (less than 1:10 mass ratio) of dwarf satellite companions over cosmic time. Artifacts of such events are preserved by the large dynamical timescales within the stellar halo, where many stellar debris can be found in abundance. While these known debris are quite diverse in morphology, they are almost uniformly comprised of old stars, which indicates these accretions are old, perhaps even ancient. Understanding the earlier phases of these accretions, when the satellite was star forming and gas rich, is left to extrapolation from their current fossilized state.RachaelBeatonCarnegie ObservatoriesOral contribution
AntitruncationsSince the measurements of the surface brightness of galaxies showed the presence of truncations the photometric properties of outer discs have been used as diagnostics for galaxy evolution, and in particular for galaxy growth. As observations became increasingly photometrically accurate the simple dichotomy between a truncated profile and a single continous exponential decline was shown to be too simple. Truncations had been detected in edge-on galaxies as an almost vertical fall-off in the profile, but in face-on galaxies they were shown to be outer zones with a steeper radial gradient than the inner zones, but still exponential. And a fraction of discs were seen not to be at all truncated down to measurable light levels. Since 2005 it has been clear that a significant fraction of disc galaxies in fact show shallower gradients in their outer discs than in their inner discs. Models incorporating inside-out growth of stellar discs plus inflow of external gas have been quite successful in explaining truncations but do not do the job either for antitruncations or single exponential discs. In this talk I will firstly review the observational situation of antitruncated discs in nearby galaxies, and go on to compare this with recent as yet unpublished work on antitruncated discs at z ~0.65 in galaxies from the GOODS-N field. I will then discuss scaling relations for antitruncations as derived in Borlaff et al. (2014) and Eliche-Moral et al. (2015). Mergers, both minor and major, have been suggested as possible formation mechanisms for antitruncated discs, as well as the natural evolution of weakly barred single galaxies. I will compare these critically, and also discuss whether antitruncations are indicative of the presence of thick discs, as observed in Spitzer images of edge-on galaxies by Comerón et al. (2014).JohnBeckmanInstituto de Astrofísica de CanariasOral contribution
The radial extent and scale-length of the Galactic thick diskUsing data from the most recent release from the Gaia-ESO Public Spectroscopic Survey I will present an investigation of the detailed structure of the Milky Way stellar disk as a function of galactocentric radius. In particular I will probe the radial extent of the Galactic thick disk and constrain its scale-length. This will give further insights into the origin of the thin and thick disks and hints if radial migration has played a major role in the evolution of the two disks or not.ThomasBensbyLund ObservatoryOral contribution
The nature and origin of substructure in the outskirts of the Andromeda galaxyWide-field surveys of the outskirts of the Andromeda galaxy have revealed much substructure, but understanding the nature and origin of this material is not straightforward from its morphology alone. I will present further constraints in the form of quantitative star formation histories for a dozen very deep HST/ACS fields probing the outer disc and substructures of M31, and discuss the results in the context of the formation of the outer disc and the effect of recent gravitational interactions.EdouardBernardObservatoire de la C¢te d'AzurOral contribution
Local Analogs for High-redshift Galaxies: Interstellar Medium Conditions and Metallicities in High-redshift GalaxiesIn the last few years, an offset between low- and high-redshift star-forming galaxies has been found in the [OIII]/Hbeta versus [NII]/Halpha BPT diagram. I will present a method to select high-redshift analog galaxies based on the location on the BPT diagram. These local analog galaxies share the same region as z~2 -3 galaxies in the BPT diagram and well resemble the properties of z~2-3 galaxies, including high specific SFRs and compact sizes, particularly, high ionization parameters and electron densities. These analogs provide a unique local laboratory to study high-redshift galaxies. I will discuss how to improve our understanding on the high-redshift metallicity empirical calibrations and potential major physical mechanism(s) to drive star-forming BPT locus evolution by using these analog galaxies.FuyanBianAustralian National UniversityOral contribution
Hot halos around spiral galaxies: a unique probe of structure formation modelsThe presence of extended hot gaseous halos around massive spiral galaxies is a fundamental prediction of all structure formation models. Yet these halos remained unexplored for several decades, thereby posing a serious challenge to observers and theorists. Although a few X-ray halos extending out to 60 kpc have been detected around nearby massive spirals based on Chandra and XMM-Newton observations, we still lack a comprehensive picture. I will present the current status on exploring hot gaseous halos, and overview how future X-ray observatories can play a crucial role in characterizing the hot halos in unprecedented details, which in turn will constrain the physical processes that play an essential role in forming galaxies from the early Universe to the present epoch.AkosBogdanHarvard-Smithsonian Center for AstrophysicsOral contribution
A new view of the spetacular 200 kpc-wide disk of the Malin 1 giant low surface brightness galaxyMalin 1 is a the iconic prototype of so-called "giant low surface brightness galaxies" (GLSBs, massive galaxies, but with low surface brightness). Around an important central concentration (that may be considered as a bulge, but is sometimes considered as a normal galaxy by itself), reside an extremely large low surface brightness disc (with a diameter larger than 200 kpc). The origin and evolution of this structure is much debated (role of interactions or collisions ? Quiescent and Star-forming phases ?), as they also are concerning the eXtended UV discs that were found by GALEX in the outskirts of "normal" galaxies. Studying the iconic case of Malin 1 will teach us about its class of galaxies (GLSBs) that are seldom studied despite their potential role in the general population of galaxies and cosmological issues. SamuelBoissierLaboratoire d'Astrophysique de Marseille (LAM)Oral contribution
Extended HI disks in nearby spiral galaxiesI will briefly review the highlights of the literature on the neutral hydrogen component in spiral galaxy outskirts, having worked on this for more than 40 years. In the 1970s I found very extended HI in galaxies such as NGC 5055, now a prototype extended UV disk galaxy, and NGC 2841, where the HI extends out to at least 2.5 time the Holmberg radius. Since these galaxies are warped, a method was devised to characterize them (the "tilted ring method"), which allows rotation curves to be derived, and evidence for dark matter to be found. The more recent work on faint stars, HII-regions, XUV disks and deep broad band optical imaging now go nearly as far out as the HI, yet for a complete understanding of galaxy outskirts, 21-cm HI line work is still a technique of choice. I will briefly sketch how this work will undergo a new revolution with the upcoming new radio facilities, leading up to the SKA.AlbertBosmaLaboratoire d'Astrophysique de MarseilleOral contribution
UV-IR color profiles of the outer regions of 2K nearby S4G galaxiesWe present our new, spatially-resolved, photometry in FUV and NUV from images obtained by GALEX, and IRAC1 (3.6 um) photometry obtained from the Spitzer Space Telescope.ƒ We analyzed the surface brightness radial profiles mu_FUV, mu_NUV, and mu_[3.6], as well as the radial evolution of the (FUV-NUV), (FUV - [3.6]), and (NUV - [3.6]) colors in the Spitzer Survey of Stellar Structures in Galaxies (S4G) nearby galaxies (closer than 40 Mpc) sample. We defined a GALEX Blue Sequence (GBS) and a GALEX Red Sequence (GBR) from the (FUV - NUV) versus (NUV - [3.6]) color-color diagram,ƒ where the GBS is populated by late-type star forming galaxies, and the GRS is populated by quiescent, early-type galaxies.ƒ While most disk becomes bluer with galactocentric distance for GBS galaxies, and stay constant for GRS galaxies, a large fraction (~50%) of intermediary GALEX Green Valley (GGV) galaxies have their outer ƒdiskƒsƒ becoming redder.ƒ This implies that star formation is suppressed in the outskirts of these galaxies,ƒ and a quenching mechanisms is needed to explain such phenomenon.ƒ We find that environmentally-related mechanisms such as starvation or ram-pressure-stripping could explain our results.AlexandreBouquinUniversidad Complutense de MadridOral contribution (DAGAL)
Molecular gas in the outer disks of spiralsThe talk will review and discuss detections and non-detections of molecular gas in the outer disks of nearby spiral galaxies. Outer disk molecular clouds have some differences with respect to their better-known inner disk counterparts, such as the mass spectrum, gas temperature, dust temperature, and possibly other properties. I will discuss the differences, how we know about them, and how clear-cut these differences are. Outer disk clouds and molecular clouds in low-metallicity galaxies have some common characteristics, but still different from inner disk clouds, and some real differences. A notable difference is the rapid transformation of molecular Hydrogen into stars at low metallicities, while outer disk clouds have a standard star formation efficiency.JonathanBraineLaboratoire d'Astrophysique de BordeauxOral contribution
HI in the outskirts of Nearby SpiralsWe analyse a subsample of galaxies from the THINGS survey to investigate the H I extent of spiral galaxy disks. We exploit the high spatial and velocity resolution combined with good sensitivity of THINGS to investigate where the atomic gas disks end and what might shape their outskirts. We find that the atomic gas surface density across most of the disk is constant at 5 to 10 Msol pc-2 and declines at large radius. The general shape of the H I distribution can be described by a Sersic-type function with a slope index, n = 0.18 - 0.36. The H I column density at which the radial profiles turn over is found to be at too high a level for it to be caused by ionisation by a meta-galactic UV field and we postulate that the H I extent is set by how galaxy disks form.EliasBrinksUniversity of HertfordshireOral contribution
Deconstructing the assembly histories and measuring the total masses of halos using discrete tracersThe SLUGGS (SAGES Legacy Unifying Globular clusters and GalaxieS) wide field chemodynamical survey of 25 + nearby, early-type galaxies, combined with the AIMSS (Archive of Intermediate Mass Stellar Systems) project, has revealed new classes of compact stellar systems, including successive record-breaking densest galaxies, that are the "smoking gun" for accretion processes. We have shown that, in many cases, these novel types of ultra compact dwarf galaxies (UCDs) are the remnant nuclei of accreted dwarf galaxies, sometimes accompanied by stellar streams containing globular clusters, whose properties provide a detailed view of satellite capture and assimilation. I will discuss how halo build up can be deconstructed from the orbital and stellar population characteristics of these bright halo tracers. I will also describe various approaches to multi-population dynamical modeling - using starlight, globular cluster subpopulations and UCDs, in concert, as independent tracers of the gravitational potential - to measure the total mass of their host galaxy out to large radius (many R_e). The relationship between relatively well-studied stellar systems (early type galaxies and star clusters) and newly discovered categories (unusually faint or dense UCDs, as well as ultra diffuse galaxies) informs our understanding of galaxy assembly processes. In addition, UCDs with central black holes shed light on the M-sigma relation and how galaxy halo (total) mass and black hole mass are interrelated. JeanBrodieUC ObservatoriesOral contribution
Evolving Sparse Stellar PopulationsWe compute the evolving spectral energy distribution of stellar populations of mass characteristic of star clusters taking into account stochastic fluctuations in the number of stars populating the IMF, and the presence of interacting binary stars in the cluster population. We evaluate under what circumstances the UV excess phenomenon is expected to appear in star clusters of different mass, and which is its most likely source: the stochastic fluctuations, the result of binary interactions, or a mixture of both. We compare the predictions of the sparse stellar population models with the standard population synthesis models over the full wavelength range, form the UV to the FIR.GustavoBruzualIRyA, UNAMOral contribution
The outskirts of massive early-type galaxies at = 0.65 in the Hubble Ultra Deep FieldThe Hubble Ultra Deep Field (HUDF) opens up an unique window to witness galaxy assembly at all cosmic distances. Thanks to its extraordinary depth, it is a privileged tool to beat the cosmological dimming, which affects any extragalactic observations and has a very strong dependence with redshift ƒ (1+z)^4. In particular, massive (M_stellar > 5x10^10 M_Sun) Early Type Galaxies (ETGs) are the most interesting candidates for these studies, as they must grow in an inside-out fashion developing an extended stellar envelope/halo that accounts for their remarkable size evolution (~5 times larger in the nearby Universe than at z=2-3). To this end we have analysed the 6 most massive ETGs at z less than unity in the HUDF12. Because of the careful data reduction and the exhaustive treatment of the Point Spread Function (PSF), we are able to trace the galaxy surface brightness profiles up to the same levels as in the local Universe but at = 0.65 (31 mag arcsec^-2 in all 8 HST bands, ~29 mag arcsec^-2 restframe or beyond 25 effective radii). This fact enables us to investigate the galactic outskirts or stellar haloes at a previously unexplored era, characterising their light and mass profiles, colors and for the first time the amount of mass in ongoing mergers. We also describe the properties of the superfaint galaxy companions that only the extreme depth and resolution of HUDF are able to unveil, even for those satellites buried under the massive galaxies' surface brightness wings.FernandoBuitragoInstitute of Astrophysics and Space Sciences (IA)Oral contribution
Finding Dwarf Galaxies From Their Tidal ImprintsThe outer HI disks of spirals are responsive tracers of tidal interactions, and can allow us to characterize CDM sub-structure and the Galactic potential. In a series of papers, we have shown that one can quantitatively characterize the mass and location of satellite galaxies from analysis of their tidal imprints on the outskirts of galaxies. I will review the proof of principle of the method, by applying it to galaxies with optically visible companions (Chakrabarti et al. 2011), as well as our earlier prediction for a new dark-matter dominated dwarf satellite galaxy of the Milky Way (Chakrabarti & Blitz 2009; Chakrabarti & Blitz 2011) for which we now have observational evidence (Chakrabarti et al. 2015). Follow-up spectroscopy (Chakrabarti et al., in prep) shows that the Cepheid candidates are moving at a radial velocity that is large and distinct from the Galaxy; the observed radial velocity is roughly consistent with the earlier dynamical prediction. This may represent the first example of Galactoseismology. While young Cepheid variables are unexpected in models of the Galactic halo, infall of gas-rich dwarf galaxies may well produce an yet undiscovered population of Cepheid variables in the outer halo. SukanyaChakrabartiRITOral contribution
The mass-metallicity relation of absorption selected high-redshift galaxiesStrong absorption lines seen in quasar spectra arise when the lines of sight to the quasars intersect intervening galaxies. The associated metal absorption lines from the strongest absorption lines, the damped Lyman alpha absorbers (DLAs), allow us to trace the metallicity of galaxies back to redshifts z>5. Typical metallicities range from 0.1-100% solar metallicities with a huge scatter at any given redshift. Understanding the nature of galaxies that host DLAs is one strategy to probe the early phase and origin of stars in the outskirts of present-day galaxy disks.LiseChristensenDark Cosmology Centre, Niels Bohr Institute, University of CopenhagenOral contribution
The dynamics of Andromeda's dwarf galaxies and stellar streamsAs part of the Z-PAndAS Keck II DEIMOS survey of resolved stars in our neighboring galaxy, Andromeda (M31), we have built up a unique data set of measured velocities and chemistries for thousands of stars in the Andromeda stellar halo, particularly probing its rich and complex substructure. In this contribution, I will discuss our efforts to dynamically and chemically characterise the dwarf galaxies and multiple stellar streams of this system, including Andromeda XIX, a dwarf galaxy located ~100 kpc from M31, that is likely disrupting; a possible extension to the most significant merger even in M31 - the Giant Southern Stream; the dynamics of other narrow stream features identified within the PAndAS survey; and how we can combine these data to refine our understanding of M31ƒ__s complex evolution. MichelleCollinsUniversity of SurreyOral contribution
Models of the outskirts of galaxies in a cosmological contextI will describe predictions for the evolution and present-day phenomenology of galaxy outskirts in the LambdaCDM model, across a very wide range of dark matter halo mass, based on studies of large-volume cosmological simulations (the Millennium II and Copernicus Complexio simulations) and high-resolution resimulations of Milky Way analogues and the most massive galaxy clusters (the Aquarius and Phoenix projects, respectively). My talk will emphasize the underlying connections between galaxy properties (such as surface brightness profiles) observed at different scales and their fundamental relationship to how galaxy formation proceeds in a LambdaCDM universe. I will contrast semi-analytic and hydrodynamical approaches to simulating diffuse starlight, including results from the Eagle simulations and a discussion of the the balance between galactic accretion and processes that may form stellar halos `in situƒ__.AndrewCooperUniversity of DurhamOral contribution
Resolving the extended stellar halos of nearby galaxies: the wide-field PISCeS surveyI will present results from the wide-field Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS): we investigate the resolved stellar halos of two nearby galaxies (the spiral NGC253 and the elliptical Centaurus A, D~3.7 Mpc) out to a galactocentric radius of 150 kpc with Magellan/Megacam. The survey led to the discovery of ~20 faint satellites and stunning streams/substructures in two environments substantially different from the Local Group, i.e. the loose Sculptor group of galaxies and the Centaurus A group dominated by an elliptical. These discoveries clearly testify the past and ongoing accretion processes shaping the halos of these nearby galaxies, and provide the first complete census of their satellite systems down to an unprecedented M_VDenijaCrnojevicTexas Tech UniversityOral contribution
Parametrizing the Stellar Halos of GalaxiesThe study of the average properties of the stellar halos of galaxies provides constraints to test our currents models of galaxy formation. To this purpose, we probe the stellar halos of central galaxies out to 70-100 kpc as a function of stellar mass (MPA-JHU), halo mass (from the Yang et al 2007 catalog) and galaxy type (concentration and surface mass density) by stacking aligned r and g band images from a large sample of galaxies from SDSS DR9. We derive surface brightness profiles to a depth of almost 31 mag/sqarcsec out to 100 kpc as well as ellipticity and g-r colour profiles. We find that the ellipticity of the stellar halo is a strong function of stellar mass and halo mass. Where the g-r colour of the stellar halo can be measured, we find that the stellar light is always bluer than in the main galaxy. We further demonstrate that the full two-dimensional surface intensity distribution of our galaxy stacks can only be fit through multi-component Sersic models. Using the fraction of light in the outer component of the models as a proxy for the fraction of accreted stellar light, we show that this fraction is a function of stellar/halo mass and galaxy type. The fraction of accreted stellar light rises from 30% to 70% and from 2% to 25% for high and low concentration galaxies respectively over the mass range 1e10 to 1e11.4 Msun. We compare our results with those of Illustris simulations. Through our stacks, we constrain how the light from the stellar halo affects the massive end of the stellar massfunction. Further, by stacking spectra of nearly 100 MaNGA early type galaxies, we detect the stellar continuum out to 3 effective radii allowing us to probe the stellar halos of these galaxies. We derive the average radial gradients of the Lick Indices out to 3 Re, which allows us to derive average metallicity and abundance gradients.RichardD'SouzaMax Planck Institute for Astrophysics;Oral contribution
Galaxy archeology with ultra deep imagingThe mass assembly of galaxies leaves various imprints in their surroundings, such as shells, streams, tidal tails, tidal dwarf galaxies, extended stellar halos, etc. The frequency and properties of these structures depend on the mechanism driving the mass assembly: rapid cold-gas accretion followed by violent disk instabilities, minor mergers or major dry / wet mergers. Therefore by studying the outskirts of galaxies, one can learn about their main formation mechanism.Pierre-AlainDucAIM Paris-SaclayOral contribution
The CARMA Project: Galaxy Formation Models' Predictions on Galaxy Formation and Cosmic FlowsOver the past two decades, numerical simulations have evolved into powerful tools for testing our understanding of the evolution of the cosmic structures. Nonetheless, existing computational limitations still require simulations to rely on phenomenological approaches when critical phenomena, such as star formation or feedback processes, involve scales that are far below resolution limits. As a workaround, several research teams (MAGICC, OWLS, EAGLE, Illustris, FIRE, etc.) have developed their own algorithms and numerical implementations to model sub-grid processes. Although considerable effort has been gone into calibrating the intrinsic parameters of individual implementations, very little work has been done, to date, to systematically compare models that are often very different both conceptually and in their technical implementation. The CARMA project aims to fill this gap. We have carried out a fair comparison of some of the most well established models of galaxy formation in order to identify their strengths and weaknesses.FabriceDurierSteward Observatory, University of ArizonaOral contribution
Dispersion in DLA metallicities and deuterium abundancesDamped Lyman-alpha absorbers (DLAs) dominate the neutral gas content of the Universe in the redshift range z=0-5 and are likely the progenitors of low redshift galaxies. Recent chemical abundance measurements of DLAs revealed a large intrinsic scatter in their metallicities. In this talk I will discuss a semi-analytic model that was specifically designed to study this scatter. This model accurately traces the chemical evolution of the interstellar matter in small regions of the Universe with different mean density, from over- to underdense regions. I will show that different histories of structure formation in these regions, namely halo abundance, mass and stellar content are reflected in the chemical properties of the proto-galaxies, and that the dispersion arising from this environmental effect is an important contribution to the overall intrinsic scatter. I will also address deuterium abundance measurements, which constitute a complementary probe of the star formation and outflow histories. I will show our predictions for the evolution of dispersion in deuterium abundance and discuss how it can be used to constrain the star formation history at high redshifts.IrinaDvorkinInstitut d'Astrophysique de ParisOral contribution
The mass distribution of S4G galaxiesWe use 3.6 um photometry for 1345 galaxies from the Spitzer Survey of Stellar Structure in Galaxies (S4G) to trace the old stellar structure of nearby disk galaxies with total stellar masses between Mƒ_*/Msun=10^8-10^11 and Hubble types between ƒT=–3-10. We aim at characterizing the stellar density profiles, the bars, and the disk contribution to the circular velocity in spiral galaxies, providing observational constraints for galaxy formation models to be tested with. We re-scale galaxy images to a common frame determined (i) by the size of their disks in physical units, (ii) by their estimated scale-length, and (iii) by both the length and orientation of the galactic bars that they host. We stack the resized images to obtain statistically representative average stellar disks and bars in bins of Mƒ_* and T. SimónDíaz-GarcíaUniversity of OuluOral contribution (DAGAL)
Stellar Populations of Close Companions and Intracluster Light around Local Brightest Cluster GalaxiesWe derive the stellar populations for a sample of local brightest cluster galaxies, resolving the BCG core, outskirts, nearby companions and intracluster light. Using spectra from the Sparsepak IFU on the WIYN telescope, throughout the BCG are the best fits to population synthesis models are found to be by old, metal rich stars, as expected. The close companions often share these characteristics, but the ICL does not share the same stellar populations as today's BCGs, as a good fit requires a component of younger and metal poor stars. Results for three representative systems, Abell 85, IIZw108 and Abell 2547 are presented.LouiseEdwardsYale UniversityOral contribution
Cold molecular halo gas in proto-cluster radio galaxies: assembling the outskirts of giant galaxies through cold accretion and jet-triggered coolingIn this talk I will present observational evidence for very extended reservoirs of cold molecular gas in the halo environments of massive proto-cluster radio galaxies at z~2. I will focus on the intriguing case of the Spiderweb Galaxy, where a large (~70 kpc) reservoir of cold molecular gas appears to fuel in-situ star-formation across the circum-galactic medium. The low velocity dispersion of the cold gas compared to that of the proto-cluster galaxies suggest that it originates from long-sought cold accretion processes. These observations directly support model predictions that the earliest evolutionary stages of giant cluster galaxies are dominated by cold gas that accretes onto the outskirts of these massive systems, where it starts forming a stellar halo. Another clear result from our study is that cold molecular gas in the halo environments of high-z radio galaxies often shows alignments with the propagating radio jets. This suggests that at high-z, powerful radio jets can induce gas cooling --and possibly even trigger star formation-- out to many tens of kpc from the host galaxy. [This work is based on extremely sensitive observations of CO(1-0) with the Australia Telescope Compact Array that were optimized to detecting low-surface-brightness emission, and also includes follow-up work done with ALMA and the VLA.]BjornEmontsCentro de Astrobiolog¡a (CSIC/INTA)Oral contribution
Constraining the dark matter content of NGC 1291 using hydrodynamic simulationsWe present a pilot study on the nearby barred galaxy NGC 1291, in which we aim to constrain the dark matter content in the inner regions of the galaxy, by breaking the disc-halo degeneracy. The potential of the galaxy is obtained from images in the near infrared, assuming different disc mass-to-light ratios (M/L), bar pattern speeds and height functions. A large number of hydrodynamic gas response simulations are run in these potentials, and the morphology of the shocks induced in the gas is compared to the morphology of the dust lanes observed in the galaxy. This allows us to constrain the three free parameters, thus providing a dynamical determination of the M/L.
The best-fit models suggest that the M/L of NGC 1291 agrees with that predicted by SPS models in the near-infrared, which leads to a borderline maximum disc for this galaxy. Furthermore, we find that the bar rotates fast, with corotation radius less than 1.4 times the bar length.
FrancescaFragkoudiObservatoire de Paris-MeudonOral contribution (DAGAL)
The observed peripheral growth of disc galaxies from z ~ 1Using images from the Hubble Space Telescope and Sloan Digital Sky Survey, we have computed both parametric and non-parametric measures, and examined the evolution in size, concentration, stellar mass, effective stellar mass density and asymmetry for a sample of 600 disc galaxies from z ~ 1 till z ~ 0. We find that disc galaxies have gained more than 50 per cent of their present stellar mass over the last 8 Gyr. Also, the increase in disc size is found to be peripheral. While the average total (Petrosian) radius almost doubles from z=1 to z=0, the average effective radius undergoes a marginal increase in comparison. The substantial increase in mass and size indicates that accretion of external material has been a dominant mode of galaxy growth, where the circumgalactic environment plays a significant role.DimitriGadottiESOOral contribution
Extended Structure of the Milky WayThe Milky Way Galaxy is now known to be enveloped by a network of stellar streams, clouds, and satellites. Many of these appear to be confined to a rather narrow plane, forming what has come to be known as the Vast Polar Structure. I describe some of these substructures and discuss the prospects for finding similar features in galaxies throughout the Local Volume.CarlGrillmairCalifornia Institute of TechnologyOral contribution
Star formation at low rates - the impact of lacking massive stars on stellar feedbackOver the recent years more dedicated observations uncovered star formation to occur in dwarf galaxies, tidal tails, ram-pressure stripped gas clouds, and the outskirts of galactic disks with extremely low rates. For such low gas conversion to stars the question must be addressed to what extent also massive stars can be formed. Observations also give hints of steeper mass funtions and upper-mass cut-offs of such stellar clusters.GerhardHenslerUniversity of ViennaOral contribution
Surfing the bar: The formation of anti-truncated stellar disksThe stellar radial profiles of disk galaxies are often observed to be truncated, or anti-truncated in the galaxies' outskirts. As of now, the literature about galaxy formation lacks a model for the formation of observed anti-truncated stellar disks which is based on secular processes. In my talk I will present an attempt to fill this gap. We were able to model anti-truncated disks in numerical SPH simulations of the formation of isolated galaxies. I will show that the stars in the outskirts of the simulated galactic disk are on very eccentric orbits but were formed on circular orbits at much smaller radii. We argue that a strong central bar is the main driver of the formation of such a disk configuration. The model predicts that such outer stellar disks should show very low rotation, but high radial dispersion. If confirmed, their existence would constitute galaxy disks of qualitatively very new kinematic properties.JakobHerpichMPIAOral contribution
On the colours of barlensesBarlenses are a very interesting type of morphological structure present in a high fraction of barred lenticular and early-type spiral galaxies. Recent works have shown that they are the more face-on counterpart of the boxy/peanut bulges observed in edge-on galaxies by means of simulations (Athanassoula et al. 2014) and observations (Laurikainen et al. 2014). This links the barlens phenomenon to the evolution of the bar and means that they form a single structure.

In this work we compare the size, orientation and colour of barlenses to the properties of the host bars. The sample of barlens galaxies is compiled from two infrared databases: the Catalogue of the Morphological Features in the S⁴G (Herrera-Endoqui et al. 2015) and the NIRS0S survey (Laurikainen et al. 2011). We estimate the size and orientation of the barlenses by fitting ellipses to points marked over a given isophotal level that describes the shape of the barlens structure. The size and orientation of the bars are taken from the respective database. The colours of the barlenses and bars are obtained using SDSS ugriz-bands data from Knapen et al. (2014).

We find that the properties of the barlenses are tightly correlated with the properties of the bars, giving additional observational support to the idea that barlenses form part of the bars and that they are the more face-on counterparts of the boxy/peanut bulges.
MartinHerrera EndoquiUniversity of OuluOral contribution (DAGAL)
What shapes stellar population profiles in the outskirts of massive, early-type galaxies?We investigate the differential impact of environment (mergers) and internal energetic phenomena on the evolution of stellar population gradients in the outskirts of massive galaxies, employing a set of high-resolved, cosmological zoom simulations. We demonstrate that outer, negative metallicity and color gradients originate from the accretion of metal-poor stellar systems. At larger radii, galaxies become typically more dominated by stellar systems accreted mostly in minor mergers. However, only strong galactic stellar-driven winds can sufficiently reduce the metallicity content of the accreted stars to realistically steepen the outer metallicity and colour gradients in agreement with present-day observations. In contrast, the gradients of the models without stellar-driven galactic winds are inconsistent with observations (too flat). Moreover, we discuss the differential impact of various AGN feedback mechanisms, such as mechanical-momentum, X-ray radiation and ionizing UV radiation, on the metallicity and colour gradients. This analysis greatly highlights the complex and superimposed impact of both energetic phenomena and environment (minor mergers) on stellar population properties in the outskirts of massive galaxies. Our results are expected to significantly contribute to the interpretation of current and up-coming IFU surveys (like MaNGA and Califa), which in turn can help to constrain models for energetic processes in simulations.MichaelaHirschmannCNRS - Institute d'AstrophysiqueOral contribution
What can the occult do for you? - Lessons from overlapping galaxy pairsInterstellar dust is still the dominant uncertainty in Astronomy, limiting precision in e.g., cosmological distance estimates and models of how light is re-processed within a galaxy. When a foreground galaxy serendipitously overlaps a more distant one, the latter backlights the dusty structures in the nearer foreground galaxy. Such an overlapping or occulting galaxy pair can be used to measure the distribution of dust in the closest galaxy with great accuracy. The STARSMOG program uses HST observation of occulting galaxy pairs to accurately map the distribution of dust in foreground galaxies in fine (less than 100 pc) detail. Furthermore, Integral Field Unit observations of such pairs will map the effective extinction curve in these occulting galaxies, disentangling the role of fine-scale geometry and grain composition on the path of light through a galaxy.BenneHolwerdaLeiden ObservatoryOral contribution
Star-formation efficiency in the outer GalaxyWe study star-formation processes in the outskirts of our Galaxy with the Galactocentric radius of > 13.5 kpc. This region has very different environments from the solar neighborhood with lower gas density and lower metallicity. In our Galaxy, star-formation rate (SFR) or constant star-formation efficiency (cSFE) for converting HI gas to stars, are derived up to Rg = 15 kpc. These values start to decrease significantly at Rg ~ 13.5 kpc, then go down to about 1/8 and 1/4 of that in the solar neighborhood, respectively, at Rg = 15 kpc. Similar trends are observed in the outskirts of other spiral galaxies. We searched for star-forming regions with WISE all-sky survey data in MIR and FCRAO outer Galaxy survey data in CO to identify new 711 star-forming regions in 240 molecular clouds up to Rg ~ 20 kpc, which enable statistical studies of star-formation processes up to the extreme outer Galaxy for the first time. Using the newly identified star-forming regions, I constructed several indices of SFE per molecular clouds for converting H2 gas to stars. As a result, we found that both indices are roughly constant throughout the outer Galaxy (Rg = 13.5 ƒ__ 20 kpc). These results suggest that star formation processes inside molecular cloud in the outer Galaxy do not heavily depend on environmental parameters. Thus, the low cSFE found in the outer Galaxy may simply reflect the lower number of molecular clouds in such region, in other words, cSFE simply depends on the amount of H2 gas in the outer Galaxy as in the inner Galaxy. Those large number of newly identified star-forming regions are valuable samples for studying detailed star formation processes in such an elusive region. A number of future follow-up observations with large telescopes should enable a comprehensive study of spatially-resolved Kennicutt-Schmidt law throughout the outskirt of our Galaxy. NatsukoIzumiInstitute of Astronomy, University of TokyoOral contribution
Studying extra-planar gas in the halos of MaNGA galaxiesUnderstanding the gas in halos of galaxies is important for shedding light on the link between gas inflow and outflow processes, star formation and the circumgalactic medium (CGM) around galaxies. By stacking spectra across about 50 galaxies observed by MaNGA, we demonstrate that we can probe the faint diffuse ionized gas past five effective radii in the halo of late-type, edge-on galaxies. We derive radial and azimuthal line luminosity profiles for Halpha, Hbeta, [OII], [OIII], [NII] and [SII] and use them to investigate the average properties of the circumgalactic gas. We find that the gas appears collimated perpendicular to the disk, and that the excitation mechanism is not shocks, but more likely UV emission from the disk. We also present trends in the line ratio and line luminosity profiles as a function of global galaxy properties such as stellar mass, bulge-to-disk ratio and disk-averaged star formation rate surface density.AmyJonesMax Planck Institute for AstrophysicsOral contribution
HST observations reveal the curious geometry of circumgalactic gasWe have discovered that warm gas only flows along galaxy major and minor axes detected out to 200 kpc. Our results are derived from a sample of HST-imaged isolated galaxies with nearby background quasars used to probe their 10^5K CGM detected in HST/COS UV spectra (traced by OVI absorption). We constrain the geometry of the gas to reside between 20-40 degrees of the projected major axis and within 60 degrees of the projected minor axis, with little-to-no gas found in between. Furthermore, strong absorption systems tend to be found along the minor axes of star-forming galaxies. We will present additional findings on the distribution of gas around galaxies provided by our unique data-set. All of our results are consistent with current view of the CGM originating from major axis-fed inflows/recycled gas and from minor axis-driven outflows.GlennKacprzakSwinburne University of TechnologyOral contribution
z ~ 6 Metal Line Absorbers as a Probe of Galactic Feedback ModelsObservations of metal absorption lines in the spectra of QSOs out to z > 6 are providing an important probe into the enrichment and ionization state of the intergalactic medium (IGM) at the tail end of reionization. In this talk I will discuss recent work using models of low- and high-ionization metal-line absorbers (OI, CII and CIV) to place constraints on galactic feedback at z ~ 6. Using simulations with four different feedback models, including the Illustris simulation, I will show how the overall incidence rate and equivalent width distribution of metal-line absorbers varies with the galactic wind scheme. I will demonstrate that the low-ionization absorbers are reasonably insensitive to the feedback implementation, with all of our models able to reproduce the observed incidence rate of OI and CII absorbers. I will show however that all models struggle to reproduce the observations of CIV, which is probing overdensities close to the mean at z ~ 6, suggesting that the metals are not being transported out into the IGM efficiently enough. Finally, I will discuss what these results could mean for the hardness of the UV background at z ~ 6. LauraKeatingInstitute of Astronomy, CambridgeOral contribution
The Structure of Halo Gas around M33Understanding the distribution of gas in and around galaxies is vital for our interpretation of galaxy formation and evolution. As part of the Arecibo Galaxy Environment Survey (AGES) we have observed the neutral hydrogen (HI) gas in and around the nearby Local Group galaxy M33 to a greater depth than the ALFALFA observations of Grossi et al. (2008). As part of this project we investigated the absence of optically detected dwarf galaxies in its neighbourhood, which is contrary to predictions of galaxy formation models. We observed 22 discrete clouds, 11 of which were previously undetected and none of which have optically detected counterparts. We find one particularly interesting hydrogen cloud, which has many similar characteristics to hydrogen distributed in the disk of a galaxy. This cloud, if it is at the distance of M33, has a HI mass of around 107 solar masses and a diameter of 18 kpc, making it larger in size than M33 itself. Additionally, we find many over-densities in the HI disk of M33, which may point towards that galaxy´s interaction history with M31.OliviaKeenanCardiff UniversityOral contribution
Spiral pattern beyond optical radius: numerical simulations and synthetic HI observationsDeep HI observations of the outer parts of disc galaxies demonstrate the frequent presence of extended, well-developed spiral arms far beyond the optical radius. To understand the nature and the origin of such outer spiral structure, we investigate the propagation in the outer gaseous disc of large-scale spiral waves excited in the bright optical disc. Using 3D hydrodynamical simulations, we show that non-axisymmetric density waves, penetrating in the gas through the outer Lindblad resonance, can exhibit relatively regular spiral structures outside the bright optical stellar disc. The amplitude of spiral structure increases rapidly with radius. Beyond optical radius, spirals become non-linear and unstable. In models for which gas is available very far out, spiral arms can extend out to 25 disc scalelengths. We also present the synthetic 21 cm data cubes extracted from various simulated models of the outer gaseous disk. We discuss numerous aspects of the spiral pattern in the gaseous periphery of galaxy disks noted in our simulations that might be interesting to compare with specific observed cases. Our synthetic 21-cm line datasets point to the existence of several significant kinematical features related to the presence of spiral pattern perturbations that should be found in deep HI observations of the galactic outskirts.SergeyKhoperskovUniversity of MilanOral contribution
Star Formation and the IMF in the Outskirts of Nearby GalaxiesThe GALEX UV satellite discovered evidence of star formation (SF) far beyond the optical edge of galactic disks (i.e. extended UV disks, or XUV disks). Star formation in the low-density outskirts is a prime target for studies of IMF variations. A truncated IMF (i.e., the absence of O stars) is suggested, but is still on debate. We present a deep H×ñ survey of 10 local XUV disks using the Subaru Prime Focus Camera (Suprime-Cam) on the Subaru 8.2m telescope. The combination of UV emission (O or B stars) and H×ñ emission (predominantly O stars) constrains the high-mass end of the IMF. We find a large number of HII regions in nearly all XUV disks as opposed to the prediction by the truncated IMF. More specifically, we find that (1) the stochastic IMF is preferred over the truncated IMF, since even some low-mass stellar clusters (10^2-3 Msun) have massive O-stars, and that (2) the standard IMF and a simple aging effect can explain the counts of UV-bright and H×ñ-bright clusters, as well as (3) their flux ratio. These suggest that massive stars are forming in the low-density outskirts as opposed to the previous expectation.JinKodaNAO Japan/Stony BrookOral contribution
Outskirts of Local Group Dwarf Galaxies Revealed by Subaru Hyper Suprime-CamLocal Group galaxies are important targets since they can be observed as assemblies of stars and their properties can be investigated in detail with a help of stellar evolutionary theory. The newly-built instrument for 8.2m Subaru Telescope, Hyper Suprime-Cam (HSC), which is 1 Giga pixel CCD camera with 1.5 degrees field of view, is the best instrument for observing Local Group galaxies. We have carried out survey for Local Group dwarf galaxies using HSC aiming to shed a light on the outskirts of these galaxies. The survey covers target galaxies out beyond the tidal radii down to the depth unexplored by previous surveys. Thanks to the high spatial resolution and high sensitivity realized by Subaru Telescope, we are able to investigate the properties of galaxies such as spatial distribution and stellar population from the very center of galaxies to the outskirts. In this presentation, I will show results for dwarf spheroidal galaxy Ursa Minor and dwarf irregular galaxy NGC6822.YutakaKomiyamaNational Astronomical Observatory of JapanOral contribution
The 2X-HI disks of spiral galaxiesThe outskirts of galaxies - especially the large hydrogen disks of galaxies - are highly susceptible to and affected by their local environment. I will highlight the giant 2X-HI disks of nearby galaxies studied as part of the Local Volume HI Survey (LVHIS), their kinematics and relation to XUV disks, local star formation and HII regions in the outer disk, and the formation of tidal dwarf galaxies. LVHIS consists of a complete sample of ~80 HI-rich galaxies in the southern sky, observed with the Australia Telescope Compact Array (ATCA) and supplemented by multi-wavelength images, which can soon be downloaded from the LVHIS homepage. If time allows, I will also present new 3D visualisations of the gas and stars in some LVHIS galaxies as obtained from the data and HI kinematic models.BaerbelKoribalskiCSIRO Astronomy and Space ScienceOral contribution
Large scale profiles of galaxies at z=0-2 studied by stacking the HSC SSP survey dataWe report the early results of the deep stacking analysis of galaxies at z=0-2 by using the very deep and wide survey data of the Hyper SuprimeCam (HSC) Strategic Survey Program (SSP) survey. To study the evolution of outskirt of galaxies, we need very deep data to detect very faint outskirts of high redshift galaxies. We studied the evolution of outskirt of galaxies by stacking the multi-band images of an extremely large sample of galaxies at z=0-2 selected from the HSC SSP survey. By stacking up to 50,000 (at this point, which will be >100,000) galaxies selected based on the photometric redshifts, we constrained the average radial profiles for 30-80 kpc of galaxies with stellar mass =10^10-11.5 M_sun at z=0-2. We also applied simple color selections for z_phot less than 1.5 galaxies to obtain the sample dominated by late or early-type galaxies. The sample dominated by late (early)-type galaxies at z20-30 kpc, stellar halo components are detected for galaxies at up to z=2. This supports the early formation of stellar halos from debris produced during hierarchical assembly of galaxies, predicted by cosmological numerical simulations. We will also report the color and surface brightness evolution of stellar halos and outer disks at z=0-2 (on-going).MarikoKuboThe University of Tokyo, ICRROral contribution
Falling outer rotation curves of star-forming galaxies at z=0.6-2.6 probed with KMOS-3D and SINS/zc-SINFThe rotation curves of star-forming galaxies at high redshift are currently well probed in their inner parts through deep IFU kinematics, while their extended shapes reaching to the outer faint levels of the disk are still largely unconstrained. I will present the results of a project examining the unexplored outer rotation curves of star-forming galaxies at high redshift, exploiting the deep H-alpha IFU kinematic data from the SINS/zc-SINF and KMOS-3D surveys. Through stacking the signal of ~100 massive galaxies at z=0.6-2.6, we are for the first time able to constrain a representative rotation curve out to several effective radii. Our stacked rotation curve exhibits a turnover with a significant decrease in rotation velocity in the outer regions, significantly strengthening the tantalizing evidence previously hinted at in a handful only of individual disks among our sample with the deepest data. These results are in good agreement with recent studies demonstrating that star-forming disks at high redshift are strongly baryon-dominated; the steep falloff of the outer rotation curve further indicates a significant level of pressure support at large radii, with important implications on the outer disk structure of massive high-redshift galaxies.PhilippLangMax-Planck-Institue for Extraterrestrial PhysicsOral contribution
Cooling and Heating in the Milky Way`s Goldilocks DiskI will discuss an effort to constrain the accretion history of the Milky Way (MW), by jointly examining the amount of disk heating (as traced by an increase in stellar velocity dispersion of the disk stars of different ages) experienced by the MW from infalling satellite galaxies, and by considering the populations of Globular Clusters that the accreted satellite galaxies might bring with them to the MW's stellar halo. Key to this exercise are new semi-analytic estimates for the energy injection of satellites in orbits of arbitrary circularity, and an empirical estimate for the velocity dispersion of the dynamically hot gas that stars in the MW disk might be born out of at high redshift. I will show that the observed Globular Cluster populations and MW disk structure, are consistent with the MW experiencing a merger history similar to those in cosmological simulations, including one or two 1:10 mass ratio accretion events - provided that the MW's gas disk was already dynamically hot at early times from internal processes (e.g., supernovae feedback and gravitational instabilities)RyanLeamanMPIAOral contribution (DAGAL)
A Deeper Look at Faint Halpha Emission in the Outskirts of Dwarf GalaxiesHow might the characteristics of Halpha emission be different in the outskirts of dwarf galaxies compared with the inner disk? I will present results from new deep Halpha imaging from the ground and with the Hubble Space Telescope, which have revealed both diffuse and structured H-alpha emission (filaments, shells, possible single- star HII regions) spanning extents up to 2.5 times larger relative to previous observations. The impact of the newly detected extended flux on our understanding of star formation, the properties of HII regions, and the propagation of ionizing photons will be discussed.JaniceLeeSpace Telescope Science InstituteOral contribution
The frequency and properties of young tidal dwarf galaxies in nearby groupsMany present-day galaxies reside in group environments where tidal interactions dominate the dynamics of the contained members. Gaseous material pulled from the outer disks of interacting spirals can produce second-generation tidal dwarf galaxies (TDGs), which differ from first-generation `classical' dwarfs by their higher metal content and lack of dark matter. I will present the results of a multi-wavelength investigation of the dwarf galaxy populations in three gas-rich interacting galaxy groups: NGC 871/6/7, NGC 3166/9, NGC 4725/47.KarenLee-WaddellCSIRO Astronomy and Space ScienceOral contribution
New Perspectives on the Gaseous Halos of Local Group GalaxiesOur view of galaxies has been transformed in recent years with diffuse halo gas surrounding galaxies that contains at least as many metals and baryons as their disks. While single sight lines through galaxy halos seen in absorption have provided key new constraints, they provide only average properties. Here, I will show that Local Group galaxies offer a new perspective and a unique opportunity to relate halo and disk phenomena. I will show recent and ongoing observational results on two local galaxies, the LMC and M31. I will describe the rationale of Project AMIGA (Absorption Maps In the Gas of Andromeda), a large approved HST program aimed to determine the spatial distribution of the halo properties of a L* galaxy using ~40 background targets at different radii and azimuths. I will discuss early results that demonstrate that M31 has a gaseous halo that 1) extends to Rvir with as much as metal and baryonic masses than in its disk, and 2) has drastic change in its ionization properties with more highly ionized gas found at R~Rvir than cooler gas found near the disk. I will show that the LMC has a massive outflow that is moving too slowly to escape the gravitational pull of the LMC on its own, and yet most of this gas is not returning onto its disk. Instead it feeds the Milky Way's halo, implying that the environment of LMC-like or dwarf galaxies may play an important role in their ultimate gas starvation.NicolasLehnerUniversity of Notre DameOral contribution
The age-kinematical features in the Milky Way outer diskThe outskirt of a disk galaxy is critically important because the secular evolution may significantly affect the properties of the disk outskirt. The outskirt of the Milky Way, in particular, can be better studied since we are able to measure the kinematics for individual stars in our Galaxy. The on-going LAMOST Milky Way spectroscopic survey has collected more than 5 million stellar spectra in the past four years, most of these data well cover the Galactic anti-center region. In this presentation, I will demonstrate the age variation of the stellar warp of the Milky Way unveiled from the LAMOST red clump stars located from 8 kpc to 14 kpc. The younger stellar warp displays larger vertical amplitude than the old one, in agreement with the misaligned gas infalling scenario of the formation of the warp. Although the tidal mechanisms of the warp formation can not be completely ruled out, it is quite challenging for these scenarios to reproduce such a age variation in the warp. The younger stellar population is also evident that its radial velocity is the perturbed by the spiral arms and hence show different bulk motions at different radii.ChaoLiuNAOCOral contribution
Neutral Gas Outside the Disks of Local Group GalaxiesThere is increasing evidence for concentrations of neutral gas that are associated with galaxies in the Local Group, yet lie well outside their stellar systems. Some can be associated with outflows, some with infall, and some has an unknown origin. I will discuss recent, extremely sensitive observations of 21cm HI emission made with the Green Bank Telescope that have detected HI clouds around M31 ~100 kpc from that galaxy's disk. The clouds have a size of a few kpc and a mass of a few 10^5 solar masses in Hydrogen, but contain no stars and do not seem to be related to the high velocity cloud population of M31. They may be products of a past interaction, condensations in the hot halo, or something else entirely.FelixLockmanNRAO Green BankOral contribution
Not Even Wrong: An Historical, Philosophical and Ultimately a Scientific Inquiry into the Schmidt LawI will rapidly review the history of the Schmidt Law and show exactly what led us to adopt the power-law formalism of a star formation rate being proportional to local gas density to some power n. I will then outline a reformulation of the problem into a cyclical process having well-defined and rate-limiting timescales and efficiencies. These basic "observables" can now be absolutely calibrated and expressed as two new "laws" of star formation that can be more directly compared to theory and simulations. Applications of this new perspective and methodology to on-going star formation in the LMC and NGC 6822 will be presented.BarryMadoreCarnegie ObservatoriesOral contribution
Stellar tidal streams in the outskirts of nearby spiral galaxiesWithin the hierarchical framework for galaxy formation, merging and tidal interactions are expected to shape large galaxies up to the present day. While major mergers are quite rare at present, minor mergers and satellite disruptions - that result in stellar streams - should be common, and are indeed seen in the halos of the Milky Way and the Andromeda galaxy. In the last years, the Stellar Tidal Stream Survey (PI. Martinez-Delgado) has obtained ultra-deep, wide-field imaging of some nearby spiral galaxies, based on data taken with a network of small robotic telescopes (0.1-0.5-meter). These images have revealed for first time external views of such stellar tidal streams at unprecedented sensitivity and detail. In this talk, I present the current results of our systematic survey of streams in the halos of nearby Milky Way-like galaxies with the ultimate aim of estimating the frequency, morphology and stellar luminosity/mass distribution of these structures in the local universe. I also discuss recent follow-up observations (e.g. Spitzer, Keck, Subaru) and N-body modeling of the most striking streams and what we can learn from the comparison of the results of our survey with L-CDM cosmological simulations of stellar halos.DavidMartínez DelgadoARI, University HeidelbergOral contribution
Outflows in low-mass galaxies at z>1Star formation histories of local dwarf galaxies, derived through resolved stellar populations, appear complex and varied. The general picture derived from hydrodynamical simulations is one of cold gas accretion and bursty star formation, followed by feedback from supernovae and winds that heat and eject the central gas reservoirs. This ejection halts star formation until the material cools and re-accretes, resulting in an episodic SFH, particularly at stellar masses below ~10^9 Msol. Such feedback has often been cited as the driving force behind the observed slowly-rising rotation curves in local dwarfs, due to an under-density of dark matter compared to theoretical models, which is one of the primary challenges to LCDM cosmology. However, these events have not yet been directly observed at high-redshift. Recently, using HST imaging and grism spectroscopy, we have uncovered an abundant population of low-mass galaxies (M*<10^9 Msol) at z=1-2 that are undergoing strong bursts of star formation, in agreement with the theoretical predictions. These Extreme Emission Line Galaxies, with high specific SFRs and shallow gravitational potential wells, are ideal places to test the theoretical prediction of strong feedback-driven outflows. Here we use deep MUSE spectroscopy to search these galaxies for signatures of outflowing material, namely kinematic offsets between absorption lines (in the restframe optical and UV), which trace cool gas, and the nebular emission lines, which define the systemic redshift of the galaxy. Although the EELGs are intrinsically very faint, stacked spectra reveal blueshifted velocity centroids for FeII absorption, which is indicative of outflowing cold gas. This represents the first constraint on outflows in M*<10^9 Msol galaxies at z=1-2. These outflows should regulate the star formation histories of low-mass galaxies at early cosmic times and thus play a crucial role in galaxy growth and evolution. MichaelMasedaLeiden ObservatoryOral contribution
Kinematics and stellar populations of NGC 3311, the cD galaxy in the center of the Hydra I clusterThe formation of extended halos around brightest cluster galaxies is related to the morphological transformation and the interactions of galaxies in clusters. In this work, we studied the stellar halo of NGC 3311 out to 30 kpc using FORS2 in MXU multi-slit mode in order to measure the large-scale kinematics and the stellar population parameters to disentangle the turbulent past and present assembly history of the Hydra cluster core.

The rising velocity dispersion profiles measured in previous studies is confirmed, indicating that the dynamics of the stars is driven by the cluster's gravitation potential and not by the galaxy halo. Moreover, the asymmetry of the velocity dispersion profile is explained by the superposition of different components along the line-of-sight.

Both the BCG and cD halo stars have old ages, but the abundance properties suggest different star formation histories, confirming the dichotomy of the origins of the stars indicated by the kinematics. On one hand, the BCG halo is well described by simple gradients, compatible with a quasi-monolithic collapse. On the other hand, the cD halo is characterized by a large spread in metallicities and element abundances, indicating that its stars were accreted from satellites of different masses.

The distribution of the metallicities and element abundances of the cD halo indicate that massive galaxies are the main progenitors of the halo stars, but contributions from low mass galaxies is also detected. These results agree with recent cosmological hydrodynamical simulations: accretions events are responsible for the formation of the extended halo, which is an on-going process occurring also at z less than unity.
ClaudiaMendes de OliveiraUniversity of Sao PauloOral contribution
Properties of the outer regions of spiral disks: abundances, colors and agesWe will present the results obtained from our evolutionary spiral models for the outer regions of disks. Our chemical and spectrophotometric models are computed for different total masses or sizes, and we will analyze the resulting radial distributions of elemental abundances, star formation, gas and stars densities and also the brighness profiles in different bands, colors, and averaged stellar ages. We will pay special attention to possible changes of slope of these distributions in the outer regions of disks and, in that case, the subjacent causes for it.MercedesMolláCIEMATOral contribution
The origin of stellar breaks at large radii as probed by the Spitzer Survey of Stellar Structure in GalaxiesWe present observational evidence that dynamically coupled bars and spirals can rearrange old stars in galactic disks out to large galactocentric distances. We used data from the Spitzer Survey of Stellar Structure in Galaxies (S4G), a deep and homogeneous survey of more than 2300 nearby galaxies imaged at 3.6 and 4.5 microns, which allows us to peer through dust at the old stellar backbone of galaxies. Using radial profiles at these wavelengths, we identified breaks in the radial distribution of old stars. We found that the distribution of the radii of breaks and bars is not random. Most breaks lie at twice the bar radius, which is the expected location of the Outer Lindblad Resonance of the bar. But we also discovered a new family of breaks at 3.5 times the bar radius. Such breaks have been predicted by simulations, and arise from a dynamical coupling between the bar and the spiral arms, when certain resonances overlap. This poses significant implications for stellar migration: under this bar-spiral coupling, disks can be thoroughly mixed out to larger radii and in shorter timescales than in the absence of bar-spiral coupling. I will also discuss our current efforts to look for breaks in the radial distribution of molecular gas in galaxies, in order to quantify how in-situ star formation and radial migration compete to build and mold the stellar structure of disks.Juan CarlosMuñoz-MateosESOOral contribution
Mass and orbital distribution of early-type galaxy haloes with the Planetary Nebula SpectrographWe present orbital anisotropy and dark halo parameters of a sample of early-type galaxies (ETGs) for which hundreds of planetary nebulae (PNe) have been observed within the PN.S Elliptical Galaxy Survey. Recent analyses have shown that PNe close follow the bulk of the stellar population and can be used in combination with long slit and integral field kinematics to constrain the mass and orbital distribution of early-type galaxies out to their outskirts (~6-10Re). Discrete tracers currently represent a pivotal tool for the investigation of these inaccessible regions of galaxies with other standard techniques, where typical dynamical times are longer and we expect to measure the signature or recent evolutionary events. In particular, the possibility to infer the variation of the orbital distribution of stars in the galaxy haloes is a strong observational test for galaxy formation scenarios than can be compared with predictions of hydrodynamical simulations. Finally, we will discuss the dark matter concentration and virial mass results against the predictions of state-of-art cosmological simulation in the LambdaCDM cosmology defined by Planck cosmological parameters. All these evidence together will allow us to gain insight on the baryon and dark matter assembly in the halo regions of early-type galaxies.Nicola RosarioNapolitanoINAF - Observatory of CapodimonteOral contribution
Vertical displacement of the Milky Way diskAn oscillating vertical displacement of the midplane of the stellar disk of the Milky Way, outside of the solar circle, has been discovered. This displacement could explain the origin of the Monoceros and Triangulum-Andromeda "Rings," in which case the stellar disk extends to a radius of at least 25 kpc. The oscillations are roughly coincident with the Milky Way's spiral arms. At a Galactic longitude of 180 degrees, the disk is perturbed up by 70 pc at 10.5 kpc from the Galactic center and 170 pc down at 14 kpc from the Galactic center. Statistical photometric parallax and the MilkyWay@home volunteer computing platform will be used to measure the perturbation of the more distant stellar disk. This perturbation of the disk could result from encounters with fairly massive dwarf galaxies or dark subhalos. The perturbations have been described as wave-like (the restoring force being provided by the gravity of the disk itself), but could instead result purely from perturbations of disk star orbits. Other formation scenarios are possible. Published results and new implications will be discussed. HeidiNewbergRensselaer Polytechnic InstituteOral contribution
Gas Kinematics in the Multiphase Circumgalactic MediumThe baryon cycle is key to understanding the observed global properties of galaxies and plays a large part in governing galaxy evolution. Red galaxies passively evolve due to star formation quenching, whereas blue galaxies actively evolve, presumably due to a replenishing gas supply. Signatures of the baryon cycle such as IGM accretion, minor mergers, and stellar driven outflows and fountains are best probed in gaseous halos, i.e., the circumgalactic medium (CGM). We use Keck, VLT, and HST spectra of background quasars to examine the kinematic properties of the multiphase, metal-enriched CGM in the outskirts of galaxies between z=0.08-1.0, focusing on the low-ionization MgII and high-ionization OVI absorption doublets. By examining the velocity dispersions of absorbers in the context of the host galaxy color, redshift, and orientation, we find that the low ionization gas more strongly traces expected baryon cycle processes, while the high ionization gas tends towards broader kinematics at larger distances from the host galaxy. We find possible effects of quenching on redder galaxies where the velocity dispersions decrease over time and are smaller at larger radii. The velocity dispersions for blue galaxies remain constant over time and radius, and possibly indicate a sustained flow of baryons feeding star formation. The velocity dispersions of absorbers located near the major axis of blue and red galaxies are comparable, indicating gas that is accreting onto or rotating around the galaxy. Blue, face-on galaxies probed along the minor axis show the largest velocity dispersions to very high significance, whereas red galaxies in this orientation show the smallest velocity dispersions. This provides evidence for multiphase galactic-scale outflows which, for this orientation, are pointing nearly towards the observer. These results place observational constraints on simulations which are just now beginning to accurately model the baryon cycle and its role in galaxy evolution.NikkiNielsenSwinburne University of TechnologyOral contribution
Formation of galaxy outskirts and origin of S‚rsic profilesThe surface-brightness profiles of galaxies are well described by the SǸrsic law: systems with high SǸrsic index m (typically giant ellipticals) have profiles steep in the centre and shallow in the outskirts, while systems with low m (typically low-mass ellipticals and spirals) have profiles shallow in the centre and steep in the outskirts. What is the origin of this connection between stellar density distribution in the outskirts and in the central regions of galaxies? A possibility is that the observed profiles arise naturally in the standard cosmological model with initial density fluctuations represented by a Gaussian random field (GRF). We explore and confirm this hypothesis with N-body simulations of dissipationless collapses in which the initial conditions are generated from GRFs with different power spectra. GRFs with much power on small scales lead to systems with high m, while GRFs with little power on small scales lead to systems with low m. In our purely dissipationless simulations the SǸrsic index is in the range 2<m<6.5. It follows that systems with SǸrsic index as low as m=2 can be produced by coherent dissipationless collapse, while high-m systems can be obtained if the assembly history is characterized by several mergers, which build dense cores at early times and extended envelopes at late times. As expected, dissipative processes appear to be required to obtain exponential profiles m=1. CarloNipotiBologna UniversityOral contribution
An extended Lyman Alpha halo at z = 3.5 seen with MUSELyman alpha haloes provide important clues to the physical properties of high-redshift galaxies outskirts. Their study allows us to probe the gas that surrounds these galaxies and to derive some of its properties (such as size, density and dust content) providing a complex context where to test models. Due to their low surface brightness, most of these studies have been performed with stacking techniques, combining the signal of a few dozens of galaxies. The new Multi Unit Spectroscopic Explorer (MUSE) provides the sensitivity to detect such halos individually. In particular, we have made use of MUSE combined with gravitational lensing magnification to study the properties of a typical (Lƒ_*, Mƒ_* = 6x10^9 Mƒsun) galaxy at z = 3.5, obtaining 2D resolved spatial information of Ly-alpha emission. The Ly-alpha emission extends over 10 kpc across the galaxy and presents a very uniform spectral profile, showing only a small velocity shift which is unrelated to the intrinsic kinematics of the nebular emission. The Ly-alpha extension is ƒ4 times larger than the continuum emission, and makes this object comparable to low-mass LAEs at low redshift. We also model the Ly-alpha line and surface brightness profile using a radiative transfer code in an expanding gas shell and compare it to the observations.VeraPatrícioCentre de Recherche Astrophysique de LyonOral contribution
Star formation and outflows in z~2 galaxiesThe last decade has seen a data deluge coming from observational facilities targeting the young universe. These data has revealed that high redshift galaxies are substantially different from their local counterpart that populates the Hubble sequence. High redshift star-forming galaxies often display clumpy morphologies associated to disk-like kinematics with a high level of turbulence. Star formation essentially occurs in these giant massive clumps and is therefore a crucial step in the life of galaxies. Reproducing the fragmentation of high redshift disk galaxies in numerical simulations is mandatory if one wants to get a realistic picture of the Hubble sequence shaping. We present state-of-the-art idealised simulations of high redshift analogue galaxies that resolve the supersonic turbulent and clumpy multi-phase interstellar medium. Our model includes a star formation criterion inspired from molecular cloud simulations which is based on a local analysis of the turbulent support of the gas clouds. The star formation efficiency associated to this approach is two order of magnitudes higher than the one using the standard density threshold method and has therefore major implications for the evolution of the galaxy. We will review through a comparative study of isolated disks and major mergers the consequences of using such a star formation criterion for the star formation history, the gas and stellar morphology of the disk as well as the properties of the galactic fountain induced by stellar feedback.ValentinPerretUniversity of ZurichOral contribution
GASP: Gas stripping and the outskirts of galaxies as a function of environmentGas inflows and outflows are fundamental drivers of galaxy evolution. We are conducting a MUSE ESO Large Program to study in detail the outskirts of 100 galaxies at z=0.04-0.1 that show optical morphologies (one-sided debris, tails and morphologies suggestive of unilateral forces acting only on the gas) that are consistent with gas being stripped and new stars being formed in the stripped gas (Poggianti et al. 2015). The MUSE IFU observations, in addition to the full main galaxy body, cover out to 60-100 kpc away from the galaxy. These galaxies have a range of galaxy masses and are found in large numbers in massive galaxy clusters, but striking candidates are also found in groups and low mass haloes. At the time of writing we have data for the first 6 galaxies, and we foresee to have ~30 galaxies (a third of the full sample) by the meeting. I would like to present the results from this first subset of the GASP results, regarding the gas stripping process, the amount of star formation involved, and the resolved kinematic and stellar population informations in the galaxies, in their outskirts and their tales.BiancaPoggiantiINAF-Astronomical Observatory of PadovaOral contribution
ICL light in a z~0.5 cluster: the MUSE perspectiveWithin a cluster, gravitational effects can lead to the removal of stars from their parent galaxies into the intracluster medium. Gas dynamical effects can additionally strip gas and dust from galaxies; both gas and stars contribute to a diffuse emission called the intracluster light (herefater ICL). The properties of the ICL can therefore constrain the types and frequency of the physical processes at work in clusters by serving as a fossil record of the interaction history of the cluster.

The present study is designed to characterize this ICL for the first time in a massive and distant cluster of galaxies, n0308, both from an imaging and a spectroscopic point of view. By applying a wavelet-based method to CFHT Megacam and WIRCAM images, we detect significant diffuse light sources and we are able to constrain their spectral energy distribution (SED). These sources were then spectroscopically characterized with ESO MUSE spectroscopic data. MUSE data also were used to compute redshifts of 24 cluster galaxies and then to search for cluster substructures.
EmanuelaPompeiESO-ChileOral contribution
What can the outskirts of galaxies tell us about dark matter?Deep observations of galaxies reveal that they are embedded in faint extended stellar structures, consisting of streams, shells, and halos. These extended stellar structures are predicted by theoretical models of galaxy formation as a natural consequence of hierarchical assembly -- gravitational tides acting on lower mass systems lead to their disruption as they interact with their more massive hosts during merging and accretion events. Because the merging timescale for the lowest mass systems is long, they tend to undergo many interactions with their host and so they are likely to spread tidal debris over the full extent of their orbit. This is interesting because it suggests that the spatial extent of extended stellar structures may differ in a systematic fashion between dark matter models in which the abundance of low-mass systems are suppressed and the canonical Cold Dark Matter (CDM) model, which is expected to have dark matter small-scale structure to effectively arbitrarily low-masses. We report on our exploration of this idea using a suite of cosmological zoom galaxy formation simulations of an individual disc galaxy and study how the properties of the extended stellar structure is likely to be influenced by the dark matter model, using both CDM and Warm Dark Matter initial conditions. We comment on the feasibility of testing this idea with future observational surveys.ChrisPowerICRAR/UWAOral contribution
Delving into the gas-phase of CALIFA galaxies to trace O and N abundance gradientsWe have studied the total oxygen abundance and nitrogen-to-oxygen ratio radial distribution using emission-lines from HII regions in a sample of 351 galaxies observed using large field-of-view PMAS optical integral field spectroscopy in the CALIFA survey. We have used the code HII-CHI-mistry (Pérez-Montero 2014) based on photoionization models, but totally consistent with the methods based on the measurement of the electron temperature. This method takes explicitly into account the N/O value to derive O/H abundances based on strong emission lines with [NII]. N/O results a much more robust indicator of the chemical status because it does not depend on ionization parameter and its relative errors are lower than for O/H, due to its large dynamical range. Our results indicate that global O/H and N/O values for all galaxies correlate between them and between other integrated properties of the galaxies (e.g. luminosity, stellar mass) much better than only using individual HII regions. A limited correlation between the resulting linear slopes of O/H and N/O is found and even a non-negligible fraction of the objects present a flat or even an inverted gradient, what can marginally depend on stellar mass or morphological type. We will explore in detail differences in the behaviour of the gradients in the galaxies outskirts using O/H and N/O and what implications this has for the calculation of the metallicity using [NII] emission lines when N/O is not considered, but [NII] are used.EnriquePé‚rez-MonteroInstituto de Astrofísica de Andalucía - CSICOral contribution
Creating lenticular galaxies with major mergersObservations of lenticular galaxies reveal a strong structural coupling between their bulges and discs, which seems difficult to reconcile with a supposedly catastrophic origin such as major mergers. I will show that this is not necessarily the case: under favourable conditions, in dissipative N-body numerical simulations of binary mergers, discs are first destroyed but soon regrow out of the debris left behind by the encounter, in full agreement with the bulge-disc coupling observed in real lenticulars soon after coalescence. Additionally, I will show how the merger scenario accounts for the recent discovery that the angular momentum in S0s is not compatible with late-type spirals. This important result from the CALIFA team imposes a serious objection to the idea that spirals transform into S0s mainly by fading, while mergers provide a satisfactory answer to such a riddle.MiguelQuerejetaMPIAOral contribution (DAGAL)
The Star Formation Rate Efficiency of Atomic-dominated Hydrogen Gas in the Outskirts of Star Forming Galaxies from z~1 to z~3Current observational evidence suggests that the star formation rate (SFR) efficiency of neutral atomic hydrogen gas measured in Damped Lyman-alpha System (DLAs) at z~3 is a factor of 10 lower than predicted by the Kennicutt-Schmidt relation. To understand the origin of this deficit, we measure the SFR efficiency of atomic gas at z~1, z~2, and z~3 in the outskirts of star forming galaxies to investigate possible coevolution with galactic properties. We use new robust photometric redshifts to create galaxy stacks in these three redshift bins, and measure the SFR efficiency by combining DLA absorber statistics with the observed rest-frame UV emission in the galaxies' outskirts. Contrary to simulations and models that predict a reduced SFR efficiency with decreasing metallicity and thus with increasing redshift, we find no significant evolution in the SFR efficiency. We conclude that the reduced SFR efficiency is driven by the low molecular content of this atomic-dominated phase, with metallicity playing a second order effect in regulating the conversion between atomic and molecular gas. This interpretation is supported by the similarity between the observed SFR efficiency and that observed in local atomic-dominated gas, such as in the outskirts of local spiral galaxies or local dwarf galaxies.MarcRafelskiNPP - NASA GoddardOral contribution
Cosmic distribution of gas and its connection with galaxiesModern state-of-the-art cosmological simulations of galaxy formation have become indispensable tools for probing the main processes that are shaping the formation and evolution of galaxies and they can be used to learn about the cycle of gas in galactic ecosystems. I will present the main results of analysing the distribution of neutral hydrogen and metals in the IGM and CGM in the EAGLE cosmological simulation (Schaye et al. 2015). I will discuss the physical properties of the HI and metal absorbers, their evolution and their connection with galaxies. I compare the simulation results with observations at high and low redshifts and discuss how this comparison can help us to learn about the role of feedback in shaping the distribution of HI and metals around galaxies.AlirezaRahmatiUniversity of ZurichOral contribution
A `Universal` Density Profile for the Outer Stellar Halos of GalaxiesThe outer stellar halos of galaxies contain vital information about the formation history of galaxies, since the relaxation timescales in the outskirts are long enough to keep the memory, while the information about individual formation events in the central parts has long been lost due to mixing, star formation and relaxation. To unveil some of the information encoded in those faint outer halo regions, we study the stellar outskirts of galaxies selected from a fully hydrodynamical high resolution cosmological simulation, called Magneticum. This simulation includes multiple sub-grid physics such as AGN and stellar feedback, metal cooling, stellar winds and an advanced model for star formation processes. In addition, improved numerical methods, including a low viscosity scheme, resulted in the self-consistent formation of both a spheroidal and a disk galaxy population at z=0 in our simulation.Rhea-SilviaRemusUniversity Observatory MunichOral contribution
The Halos and Environments of Nearby Galaxies (HERON) SurveyWe have used a dedicated 28-inch f/3.2 prime focus telescope to image ~200 nearby galaxies, largely dominated by the 2MASS Large Galaxy Atlas to a surface brightness of 30 mag/sq arcsec. We have supplemented the 2MASS sample with dwarf galaxies and distant luminous galaxies. Low surface brightness halos exceeding 50 kpc in diameter are found only in galaxies more luminous than L*, and classic interaction signatures are relatively infrequent. Halo diameter is correlated with total galaxy luminosity. Extended low surface brightness halos are present even in galaxies as faint as M_V=-18. Edge-on galaxies with boxy bulges tend to lack extended spheroidal halos, while those with large classical bulges have extended round halos, supporting the notions that boxy or barlike bulges originate from disks. Most face-on spiral galaxies present features that appear to be irregular star forming extensions, although rare cases show smooth outer halos with no sign of star formation. We will discuss plans to compare the data with theoretical models, and to explore the low surface satellites of nearby galaxies.R. MichaelRichUCLAOral contribution
Hot gas in Milky Way size galaxies at z=0We present a new set of cosmological Milky Way size galaxy simulations using ART. In our simulations the main system has been evolved inside a 28 Mpc cosmological box with a spatial resolution of 109 pc. At z=0 our systems have an Mvir = 6-8·10^11 Msun. In several of out models we have observed how a well defined disk is formed inside the dark matter halo and the overall amount of gas and stars is comparable with MW observations. Several non-axisymmetric structures arise out of the disk: spirals, bars and also a warp. We have also observed that a huge reservoir of hot gas is present at large distances from the disk, embedded in the dark matter halo region, accounting for only a fraction of the "missing baryons". Gas column density, emission (EM) and dispersion (DM) measure have been computed from inside the simulated disk at a position of 8 kpc from the center and in several directions. Our preliminary results reveal that the distribution of hot gas is non-isotropic according with observations Gupta et al. 2012 and Gupta et al. 2013. Also its metallic content presents a clear bimodality what is a consequence of a recent accretion of a satellite galaxy among others. After a careful analysis we confirm that due to the anisotropy in the gas distribution a new observational parameter needs to be defined to recover the real distribution of hot gas in the galactic halo (Roca-Fàbrega et al. 2016, submitted).SantiRoca-FabregaHebrew University of JerusalemOral contribution
Dense Cloud Cores revealed by ALMA CO observations in the low metallicity dwarf galaxy WLMUnderstanding stellar birth requires observations of the clouds in which they form. These clouds are dense and self-gravitating, and in all existing observations, they are molecular with H2 the dominant species and CO the best available. When the abundances of carbon and oxygen are low compared to hydrogen, and the opacity from dust is also low, as in primeval galaxies and local dwarf irregular galaxies CO forms slowly and is easily destroyed, so it cannot accumulate inside dense clouds. Then we lose our ability to trace the gas in regions of star formation and we lose critical information on the temperatures, densities, and velocities of the material that collapses. I will report on high resolution observations with ALMA of CO clouds in the local group dwarf irregular galaxy WLM, which has a metallicity that is 13% of the solar value and 50% lower than the previous CO detection threshold and the properties derived of very small dense CO clouds mapped.MonicaRubioUniversidad de ChileOral contribution
Chemical Abundaces of Seven Outer Halo M31 Globular Clusters from the Pan-Andromeda Archaeological SurveyObservations of stellar streams in M31's outer halo suggest that M31 is actively accreting several dwarf galaxies and their globular clusters. Detailed abundances can chemically link clusters to their birth environments, establishing whether or not a globular cluster has been accreted from a satellite dwarf galaxy. This talk presents the detailed chemical abundances of seven M31 outer halo globular clusters (with projected distances from M31 greater than 30 kpc), as derived from high resolution integrated light spectra taken with the Hobby Eberly Telescope. Five of these clusters were recently discovered in the Pan-Andromeda Archaeological Survey (PAndAS)---this paper presents the first determinations of integrated Fe, Na, Mg, Ca, Ti, Ni, Ba, and Eu abundances for these clusters. Four of the target clusters (PA06, PA53, PA54, and PA56) are metal-poor ([Fe/H] less than -1.5), alpha-enhanced (though they are possibly less alpha-enhanced than Milky Way stars at the 1 sigma level), and show signs of star-to-star Na and Mg variations. The other three globular clusters (H10, H23, and PA17) are more metal rich, with metallicities ranging from [Fe/H] = -1.4 to -0.9. While H23 is chemically similar to Milky Way field stars, Milky Way globular clusters, and other M31 clusters, H10 and PA17 have moderately low [Ca/Fe], compared to Milky Way field stars and clusters. Additionally, PA17's high [Mg/Ca] and [Ba/Eu] ratios are distinct from Milky Way stars, and are in better agreement with the stars and clusters in the Large Magellanic Cloud (LMC). None of the clusters studied here can be conclusively linked to any of the identified streams from PAndAS; however, based on their locations, kinematics, metallicities, and detailed abundances, the most metal-rich PAndAS clusters H23 and PA17 may be associated with the progenitor of the Giant Stellar Stream, H10 may be associated with the SW Cloud, and PA53 and PA56 may be associated with the Eastern Cloud. CharliSakariUniversity of WashingtonOral contribution
Gas accretion from the cosmic web feeding disk galaxiesDisk galaxies in cosmological numerical simulations grow by accreting gas from the cosmic web (e.g., Dekel+09). This gas reaches the external disk, and then spirals in dragged along by tidal forces and/or disk instabilities (e.g., Ceverino+16). The importance of gas infall is as clear from numerical simulations as it is obscure to observations (SA+14). Extremely metal poor galaxies seem to be the best example we have of the gas accretion process at work. They have large off-center starbursts which show significant metallicity drop compared with the host galaxy. This observation is naturally explained as a gas accretion event catch in the act (e.g., SA+15). After a brief contextual introduction, we will present kinematic properties of some of the metal poor starbursts, which suggest that the starbursts are independent objects traveling inward within the disk. We will discuss this result in the context of numerical simulations of cold flow accretion.JorgeSanchez AlmeidaInstituto de Astrofisica de CanariasOral contribution
The MUSE QSO Blind Survey: A Census of Absorber Host GalaxiesUnderstanding the distribution of gas in galaxies and its interaction with the IGM is crucial to complete the picture of galaxy evolution. At all redshifts, absorption features seen in QSO spectra serve as a unique probe of the gaseous content of foreground galaxies and the IGM, extending out to ~200kpc. Studies show that star formation history is intimately related to the co-evolution of galaxies and the IGM. In order to study the environments traced by absorption systems and the role of inflows and outflows, it is critical to measure the emission properties of host galaxies and their halos. We overcome the challenge of detecting absorption host galaxies with the MUSE integral field spectrograph on VLT. The large field of view provided by MUSE and its sensitivity to emission lines has allowed a never-before seen match between the number density of absorbers along QSO sightlines and the number density of emission line galaxies within 200 kpc of the QSO. These galaxies represent a sample for which previously elusive connections can be made between mass, metallicity, SFR, and absorption.LorrieStrakaLeiden ObservatoryOral contribution
The buildup of the outskirts of distant star-forming galaxies at z~2At the peak of the cosmic star formation rate at z~2, galaxies underwent intense and rapid evolution in morphologies. I present a sample of 35 star-forming galaxies on the star-forming Main Sequence at z ~ 2 from the SINS/zC-SINF survey, which gives us an unprecedented resolved view on the buildup of the galaxiesƒ__ centers and outskirts. For these galaxies, we use the powerful combination of the HST WFC3 and ACS imaging of the young and old stellar components and SINFONI/AO data of the dynamics and recent star formation activity, both with kpc-scale resolution and out to 3-4 disk scale lengths. Using this data, we constrain the spatially resolved information on the star formation rates in these galaxies on three different timescales: 10^7 yr (H-alpha maps), 10^8 yr (rest-frame UV maps) and 10^9 yr (rest-frame optical maps). In this talk, I will focus on the build up of the stellar component of the galaxies by determining the star formation histories of the individual star-forming clumps and the interclump regions in order to understand the lifetime of the clumps and the origin of the disk stars in the outskirts of the galaxies.SandroTacchellaETH ZurichOral contribution
The missing satellite problem outside of the Local GroupThe LCDM is now widely accepted as a standard cosmological model. It has been thoroughly tested on large-scales by massive spectroscopic surveys such as the Sloan Digital Sky Survey. However, there are a few possible flaws on small-scales. One such possible flaws is the missing satellite problem - LCDM predicts far too many subhalos than the observed number of dwarf galaxies around the Milky Way or in the Local Group (LG). Unfortunately, the problem has been addressed only within the LG so far based on a BIG assumption that the LG is typical halo in the Universe. However, any cosmological problems must be addressed with a statistical rigor and here we present our on-going effort to perform the first statistical test of the problem using galaxies outside of the LG. We have carried out a pilot observation of several nearby galaxies with Hyper Suprime-Cam, a recently commissioned wide-field imager on the Subaru Telescope. Thanks to the deep and wide data taken under excellent seeing conditions, we have identified a number of dwarf galaxies around the nearby galaxies. Our first results from the pilot run are (1) the observed number of dwarf galaxies is smaller by a factor of 2 than recent hydro-dynamical simulations, and (2) there seems to be a large halo-halo variation in terms of the abundance of dwarf galaxies both in observation and simulation. These results highlight the need for a larger sample and we discuss future prospects of our project. In addition to the missing satellite problem, our data allow us to address other possible small-scale flaws such as the satellite alignment problem and we report our preliminary results on it as well.MasayukiTanakaNational Astronomical Observatory of JapanOral contribution
Dynamical Structure and Evolution of Nuclear Star ClustersNuclear Stellar Clusters (NSCs) are commonly observed in the centers of most galactic nuclei. While their study can reveal important information about the dynamical structure of the innermost regions of galaxies, their formation is still poorly understood. NSCs might have been formed through gas infall and in situ star formation, or through the infall of multiple stellar clusters in the center of the galaxy. We investigate the cluster infall scenario by using N-body simulations of infalling clusters to the nucleus of a Milky Way - like galaxy. We find that the resulting NSC can show a significant amount of rotation and flattening that makes it comparable with observations of the Milky Way NSC. We study the kinematic evolution of the NSC, in order to see how the merger history can imprint fossil records on its dynamical structure. This study shows the plausibility of the cluster infall scenario, and can help towards setting better constraints to the formation history of observed NSCs.AthanasiaTsatsiMax Planck Institute for AstronomyOral contribution
MESSIER: a satellite to uncover the ultra-low surface brightness universeThe measurement of ultra-low surface brightness levels requires an exquisite control over systematics: sky variability, flat field accuracy, wings of the PSF, and straylight contamination. In addition, the proper removal of foregrounds (zodiacal light, dust cirri) can only be achieved with multi-wavelength observations. MESSIER is a satellite designed with these constraints in mind. The aim of the orbiter, which was submitted to ESA-CAS as a small-class satellite, is a multiband mapping of the full sky at SB levels of 33 mag /arcsec2 (950 nm) to 37 (200 nm) in 8 filters with 1 arcsec pixels. These unprecedented SB levels allow the exploration of science cases unachievable from the ground: the light profiles of galaxies and their putative edges, the characterisation of stellar populations within nearly half a virial radius, the detection of satellites and substructures, the warm molecular content of galaxies, the properties of intra-cluster light, and the fluctuations of the UV/optical cosmological background.DavidValls-GabaudIoA Cambridge / Observatoire de ParisOral contribution
Modeling the UV spctral rangeWe present new stellar population models in the NUV spectral range. The models provide good fits to both colours and line-strengths of early-type galaxies, which are fully consistent with the results obtained in the optical range. These fits point to the presence of very small contributions from intermediate-aged stellar populations. We conclude that the NUV shows an unprecedented ability to disentangle young stellar contributions that are not detected in the visible, which makes it very useful to study galaxy gradients.AlexandreVazdekisInstituto de Astrofísica de CanariasOral contribution (DAGAL)
Disentangling the intragroup HI in Compact Groups of galaxies by means of X3D visualizationAs an extreme kind of environments, Hickson Compact groups (HCGs) have shown to be very complex systems. HI-VLA observations revealed an intrincated network of HI tails and bridges, tracing pre-processing through extreme tidal interactions. CGs seem to be evolving from a phase where the gas is located in the galaxy disks, to intermediate cases where the HI is mostly found in the intragroup medium, and finally into a stage where almost no HI is detected (Verdes-Montenegro et al 2001). Comparison with high-quality GBT observations (Borthakur et al 2010, 2014) has provided evidence for the existence of a diffuse HI component missed by the VLA that increases with evolutionary stage, spread over a velocity range of more than 1000 km/s. This suggests that slow evolution of tidal debris may lead to a final stage where the HI becomes faint and extended (hence escaping detection by current interferometers) being returned to the IGM. In summary, the complex net of detected HI seems so puzzling as the missing one.

While SKA1 starts observations, further progress can be made through a) studies at complementary wavelengths (e.g. Tzanavaris et al 2010, Cluver et al 2013, Alatalo et al 2014), b) preparatory work through pathfinders, and c) advanced visualization techniques that allow improving significantly the insight into as complex datasets as the HI cubes of these strongly interacting systems.

In this talk we will revisit the existing VLA information on the HI distribution and kinematics of HCGs by means of X3D visualization. The X3D pathway (Vogt et al 2015) constitutes both a powerful tool to extract the most from HI data cubes and as a mean of simplifying and easing the access to data visualization and publication via three- dimensional (3-D) diagrams. These elements, in turn, can help paint a finer view of the evolution of the cold gas in group environments.
LourdesVerdes-MontenegroInstituto de Astrofísica de Andalucía (CSIC)Oral contribution
Hunting for the intrinsic scatter in the Tully-Fisher relationNeutral hydrogen disks in galaxies typically extend far into the dark matter halos of galaxies. They are kinematically cold and highly susceptible to physical processes that play an important role in the evolution of galaxies such as tidal interactions, minor mergers, ram-pressure stripping and cold accretion. The upcoming blind HI imaging surveys with Apertif on the WSRT will detect HI in 10^5 galaxies and resolve the HI disks in 10.000 galaxies, revealing the relative importance of these processes in various cosmological environments. I will briefly discuss this topic and the HI surveys that are currently being planned for Apertif.MarcVerheijenKapteyn InstituteOral contribution
Circumgalactic PrecipitationGalaxies are commonly thought to acquire much of the gas that fuels star formation through streams of cold gas that flow along filaments of larger-scale structure. However, the universe's largest galaxies appear to have a different gas supply: precipitation of cold clouds out of hot circumgalactic gas via thermal instability. I will present both observational and theoretical support for the precipitation mode in large galaxies and show how the precipitation model can be applied to galaxies of all masses. One of the attractive features of the precipitation model is that it makes observationally testable predictions about the state of the circumgalactic medium around star-forming galaxies that are in a precipitating state.MarkVoitMichigan State UniversityOral contribution
The Connection between Molecular Gas and Star Formation in XUV DisksStudying star formation beyond the optical radius of galaxies allows us to test empirical relations in extreme conditions with low average gas density and low molecular fraction. Previous studies discovered galaxies with extended ultraviolet (XUV) disks, which often contain star forming regions with lower Halpha-to-far-UV (FUV) flux ratios compared to inner disk star forming regions. However, most previous studies lack measurements of molecular gas, which is presumably the component of the interstellar medium out of which stars form. We analyzed published CO measurements and upper limits for fifteen star forming regions in the XUV or outer disk of three nearby spiral galaxies and a new CO upper limit from the IRAM 30 m telescope in one star forming region at 3.4 times the optical radius in the XUV disk of NGC 4625. We found that the star forming regions are in general consistent with the same molecular-hydrogen Kennicutt-Schmidt law that applies within the optical radius, independent of whether we used Halpha or FUV as the star formation rate (SFR) tracer. However, a number of the CO detections are significantly offset towards higher SFR surface density for their molecular hydrogen surface density. I will discuss how a deep, kiloparsec-scale CO map may enable us to use the presence or absence of molecular gas as an evolutionary probe to break the degeneracy between age and stochastic sampling of the initial mass function as the explanation for the low Halpha-to-FUV flux ratios in XUV disks.LindaWatsonEuropean Southern Observatory, ChileOral contribution
Using rotation measure to search for magnetic fields around galaxies at z~0.3Magnetic fields are an important component in galaxies. They accelerate cosmic rays, affect star formation, and regulate the redistribution of matter and energy within galaxies. Despite their ubiquity, we still do not know how these magnetic fields were originally seeded within galaxies, nor how they have grown to the strengths we observe today. One way we can unravel this complex problem is by measuring the growth of magnetic fields over cosmic time. We present the initial results of a rotation measure study to search for the presence of coherent magnetic fields around young disk-like galaxies at z~0.5. The S-band receiver at the VLA allows us to simultaneously observe Stokes I, Q, U, and V from 2-4 GHz. With these broadband polarization observations we apply multiple methods for determining the rotation measure of each source, improving the fidelity of our results. Our study contains 38 quasars with a single MgII absorption line at 0.38<z<0.6 identified by the SDSS, and over 100 quasars without MgII absorption that serve as a control sample to constrain the rotation measure intrinsic to quasars. Beyond magnetogenesis, the results of this study also have implications for the life-cycle of baryons within galaxies and the composition of galactic haloesƒ__are magnetic fields expelled with galactic winds, do these magnetic fields reach the IGM, is there a dynamo action in the haloes of galaxies? AnnaWilliamsUniversity of Wisconsin-MadisonOral contribution
The spatially-resolved study of initial mass function in the outer GalaxyWe are studying star formation in the outskirts of our Galaxy with the Galactocentric distance (Rg) of >~15kpc and the distance of >~7kpc. The outer Galaxy is known to have different environments than the solar neighborhood, e.g., low metallicities ([M/H] ~ -1dex) and low gas densities, which are similar to outskirts of other spiral galaxies and dwarf irregular galaxies. Among them, the outer Galaxy is so far the only site for studying resolved stellar populations on the same basis of solar neighborhood due to the small distances. We have obtained NIR images for ~10 star-forming clusters in the outer Galaxy using Subaru 8.2-m telescope, and cluster members are clearly resolved with mass detection limits of the order or less than 0.1Mo. With the resolved members for several clusters, IMF in the outer Galaxy is suggested to be consistent with that for the solar neighborhood in terms of both high-mass slope and IMF peak. Upcoming observations with higher spatial resolution and sensitivity, using e.g., TMT and JWST, will allow us to extend spatially-resolved study to Local Group galaxies.ChikakoYasuiUniversity of TokyoOral contribution
GHOSTSThe stars in the halos around galaxies are thought to be for a significant fraction the result of tidally disrupted infalling smaller galaxies, an important process in the mass build up of galaxies. However, in recent year the realization has grown that a fraction of halo stars may have formed in situ or in the disk of the host galaxy.

To understand the nature of stellar halos, the GHOSTS project has used the Hubble Space Telescope to observe the resolved stellar populations in the outskirts of 18 nearby massive galaxies, with a range of masses and seen from face-on to edge-on. From two filter observations (F814W and F606W) with both WFC3 and ACS, we map the different giant populations in these haloes: typically Asymptotic, Red, and He-burning Giants (AGB, RGB and HeB). By studying the distribution of these stellar populations, the GHOSTS project aims to characterize on a statistical basis the size, shape, age, amount of sub-structure, and chemical composition of stellar halos, thereby allowing us to constrain models of the hierarchical galaxy formation process.

In this presentation we will in particular show the structural parameters of the observed stellar halos out to 70 kpc. We find a surprisingly large population of AGB stars with ages less than 2 Gyr seen to ~20 kpc above the disks of many edge-on galaxies and speculations on the origin of these young populations at a location where no current star formation is seen.
Roelofde JongLeibniz-Institut fuer Astrophysik (AIP)Oral contribution
Results from the EDGES SurveyI will present results from our recent deep imaging survey with the Spitzer Space Telescope designed to identify and measure the faint stellar populations around nearby galaxies. The Extended Disk Galaxy Exploration Science (EDGES) Survey includes a sample of 92 nearby galaxies with a range of morphological types and environments. The observations include a field-of-view of at least 5 times the optical size and are deep enough to detect stellar mass surface densities of several hundredths of a solar mass per square parsec. The observations reveal extended stellar features, such as stellar disks and stellar streams, around many of the target galaxies, as expected from hierarchical galaxy formation scenarios. I will present an overview of the EDGES Survey and highlight a few of its key results.Liesevan ZeeIndiana UniversityOral contribution
Star formation efficiency in the outer disks of spiral galaxiesIn order to quantify the relationship between gas accretion and star formation, we analyse a sample of 29 nearby galaxies from the WHISP survey. We compare combined radial profiles of FUV (GALEX) and IR 24 ׬m (Spitzer) characterising distributions of recent star formation with radial profiles of CO (IRAM, BIMA, or CARMA) and HI (WSRT) tracing molecular and atomic gas contents to examine star formation efficiencies in symmetric (quiescent), asymmetric (accreting), and interacting (tidally disturbed) galaxies. In addition, we investigate the relationship between star formation rate and HI in the outer discs for the three groups of galaxies. We confirm the general relationship between gas surface density and star formation surface density, but do not find significant differences between the three groups of galaxies. The star formation efficiency in the outer parts is remarkably constant, suggesting the outer disks are just about Q-stable.Thijs (J.M.)van der HulstKapteyn Astronomical Institute, University of GroningenOral contribution