Abstracts

Monday, July 9, Talks

Session I: Overview of Groups and Clusters and Environmental Effects

The Roles of Clusters and Groups in Galaxy Evolution

Ann Zabludoff
U. Arizona/Steward Observatory

It has been more than 30 years since the publication of the morphology-density relation and the Butcher-Oemler effect. Yet still we have not fully defined the role of environment in galaxy evolution. I will review the evidence that nurture by environment is critical for transforming galaxies, but that clusters are not essential drivers of change. Mechanisms present in the building-blocks of clusters, poor groups of galaxies, appear to matter more. New observations of groups suggest that galaxy tidal interactions and mergers alter morphologies and star formation histories, as well as strip stars, dark matter, and gas from group galaxies. Over time, the brightest group galaxy grows and moves toward the group center, the ratio of dwarf-to-giant galaxies increases, and some group members evolve from late to early types. The growth of clusters from such poor groups, instead of conditions specific to clusters, is thus likely to determine the observed galaxy populations of clusters.

Evidence for Environmental Effects in Star-Forming Galaxies

Martha Haynes
Cornell University

Virtually all star-forming galaxies contain neutral hydrogen and many galaxies, especially low mass ones, contain more mass in HI than in stars. While star formation is more directly linked to the molecular clouds, the HI content reflects the fertility of a galaxy for future star formation. As evidenced by the severe gas deficiency of galaxies in dense regions, the cluster environment proves hostile to the fragile interstellar medium in the outer disks of galaxies. In this talk, I will review the evidence for gas deficiency, its relation to star formation, and the likely mechanisms which are responsible for it.

The Effect of Environment on the Molecular Gas Content of Nearby Galaxies

Christine D. Wilson and J. Golding
McMaster University

We are carrying out a comparison of the molecular gas content in isolated, group, and Virgo cluster galaxies using new CO data from the JCMT Nearby Galaxies Legacy Survey (NGLS). The NGLS is an HI-selected sample of 155 galaxies with distances < 25 Mpc covering all morphological types. Even though our isolated and group subsamples have very similar HI mass and distance distributions, we find an unusually low CO detection rate among the isolated galaxies compared to the group galaxies. The isolated galaxies also have lower K-band luminosities per unit mass of HI gas compared to the galaxies in groups, which suggests that they have been relatively inefficient at forming stars. Another interesting result is that the few ellipticals detected in CO are all found in group environments rather than being isolated or in the Virgo cluster.

Session II: X-ray Properties of Groups and Clusters

Hot Gas and AGN Feedback in Nearby Groups and Clusters

Christine Jones
Center for Astrophysics

X-ray observations of the hot gas in clusters, groups and galaxies can be used to determine the system's dynamical state, the energy produced by outbursts from supermassive black holes, as well as the mass and distribution of the hot gas and the total system mass. Gas can be lost from galaxies through ram pressure stripping during galaxy infall into clusters or through galactic winds, driven by either AGN outbursts or supernovae. The effects of ram pressure stripping are best observed in nearby clusters, particularly in Virgo, Fornax, Coma and A3627 where long tails of stripped hot gas trail infalling elliptical and spiral galaxies. The effects of AGN outbursts are seen in the hot atmospheres of early-type galaxies, groups and clusters. These outbursts, fueled by the infall of cooling gas onto the supermassive black hole, likely reheat the cool gas in the cores of galaxies, groups and clusters and limit new star formation, thus maintaining the separation of galaxies into their two fundamental classes -- the red and dead early type galaxies with little star formation and the blue cloud of spiral, actively star forming galaxies.

This presentation reviews the overall X-ray properties of clusters groups and galaxies, focussing on cooling cores and associated AGN outbursts, the stripping of hot gas during galaxy infall into clusters, hot gas atmospheres around massive spiral galaxies and around optically faint and optically bright early type galaxies.

Intergalactic Gas in Groups of Galaxies: Implications for Dwarf Spheroidal Formation and The Missing Baryons Problem

Emily Freeland & E. Wilcots
Texas A&M University

Radio galaxies with bent jets are predominantly located in groups and clusters of galaxies. We use bent-double radio sources, under the assumption that their jets are bent by ram-pressure, to probe intragroup medium (IGM) gas densities in galaxy groups. This method provides a direct measurement of the intergalactic gas density and allows us to probe IGM gas at large radii and in systems whose IGM is too cool to be detected by the current generation of X-ray telescopes. We find gas with densities of 10^-3-10^-4 per cubic centimeter at group radii from 15-700 kpc. A rough estimate of the total baryonic mass in intergalactic gas is consistent with the missing baryons being located in the IGM of galaxy groups. The neutral gas will be easily stripped from dwarf galaxies with total masses of 10⁶-10⁷ solar masses in the groups studied here. Indications are that IGM gas densities in less-massive systems like the Local Group should be high enough to strip gas from dwarfs like Leo T and, in combination with tides, produce dwarf spheroidals.

Cool core properties of galaxy groups-Details on feedback and star formation

Vijaysarathy Bharadwaj
Argelander Institut für Astronomie, Bonn, Germany

We present results for a luminosity limited sample of 26 galaxy groups with Chandra data and radio catalog data, to discern ICM cooling and AGN feedback on galaxy group scale. The cool core, non-cool core nature is distinguished using the central cooling time. Comparisons with properties of galaxy clusters are made and surprising and unexpected differences are found and discussed. A certain uniqueness vis-a-vis the temperature profiles of fossil groups is also shown and discussed with respect to the cool core paradigm. Lastly, preliminary results for star formation of some of the group BCGs are presented and we present a complete picture of the gas cooling, star formation and feedback.

Residual Cooling and Persistent Star Formation amid AGN Feedback in Abell 2597

Chris O'Dea, G. Tremblay, S. Baum, T. Clarke, R. Mittal
Rochester Institute of Technology

New Chandra X-ray observations combined with existing radio, optical, UV, and IR data enable a multi-wavelength study of active galactic nucleus (AGN) heating and intracluster medium (ICM) cooling in the brightest cluster galaxy (BCG) of Abell 2597 (z = 0.0821). The new Chandra observations reveal the central < 30 kiloparsec X-ray cavity network to be more extensive than previously thought, and associated with enough enthalpy to theoretically inhibit the inferred classical cooling flow.

Nevertheless, we present new evidence, consistent with previous results, that a moderately strong residual cooling flow is persisting at 4%-8% of the classically predicted rates in a spatially structured manner amid the feedback-driven excavation of the X-ray cavity network. The cooling time profile of the ambient X-ray atmosphere is used to map the locations of the observational star formation entropy threshold as well as the theoretical thermal instability threshold. Both lie just outside the < 30 kpc central region permeated by X-ray cavities, and star formation as well as ionized and molecular gas lie interior to both. The young stars are distributed in an elongated region that is aligned with the radio lobes, and their estimated ages are both younger and older than the X-ray cavity network, suggesting both jet-triggered as well as persistent star formation over the current AGN feedback episode. Bright X-ray knots that are coincident with extended Ly-alpha and FUV continuum filaments motivate a discussion of structured cooling from the ambient hot atmosphere along a projected axis that is perpendicular to X-ray cavity and radio axis. We conclude that the cooling ICM is the dominant contributor of the cold gas reservoir fueling star formation and AGN activity in the Abell 2597 BCG.

Session III: Compact and Loose Groups

Environmental Effects in Compact and Loose Groups of Galaxies

Hernán Muriel, Valeria Coenda & H. Julian Martínez
OAC-IATE. Córdoba, Argentina

We compare the properties of galaxies in compact groups, loose groups and in the field to deepen our understanding of the physical mechanisms acting upon galaxy evolution in different environments. We select samples of galaxies in compact groups identified by McConnachie et al., loose groups identified by Zandivarez & Martinez, and field galaxies from the Sloan Digital Sky Survey. We analyse a number of galaxies properties including: absolute magnitude, stellar mass, colour, size, surface brightness and concentration. We also study the colour-luminosity and the luminosity-size relations. Among the environments considered, compact groups have the highest fraction of red and early-type galaxies. On average, galaxies in compact groups are systematically smaller, more concentrated and have higher surface brightness than their counterparts in the field and in loose groups. Our results suggest that the physical mechanisms that transform late type galaxies into earlier types could be more effective within compact groups given the high densities and low velocity dispersion that characterise that particular environment.

Molecular Gas and Star Formation in Hickson Compact Groups: Enhanced or Deficient?

Ute Lisenfeld, V. Badenens, D. Espada, L. Verdes-Montenegro
Universidad Granada

Galaxies in HCGs can show extreme deficiencies in their atomic gas content. How does this deficiency - and the extreme environment in general - affect their molecular gas content and their star formation properties. In order to answer this, we have studied the molecular gas content and the star formation rate (SFR traced by the FIR emission) in galaxies in a sample of 20 Hickson Compact Group (HCGs) for which HI VLA data is available. The groups show a wide range in deficiency in HI and allow us to compare the molecular gas content and SFR to the HI properties. We used a sample of isolated galaxies from the AMIGA project as a comparison sample. We found that galaxies in HCGs, in contrast to their sometimes extreme HI deficiencies, show on average neither a large deficiency in molecular gas nor in the star formation rate. We find some indications for a possible enhancement of the molecular gas content. Also, the star formation efficiency is similar to that in isolated galaxies. Only the specific SFR (i.e. SFR/stellar mass) shows a trend to increase for lower HI or H₂ deficiencies.

A UV-X-ray Census of AGN activity and Star Formation in Compact Groups

Panayiotis Tzanavaris
NASA/GSFC/Johns Hopkins

We present a combined Chandra/X-ray - Swift/ultraviolet study of nuclear activity and global star formation properties in a sample of 9 compact groups that includes ~40 galaxies. This is the highest resolution and most detailed systematic study of this type ever performed in this important environment. Our results address two main issues, namely (1) the level of AGN activity and (2) star-formation in compact groups, compared to clusters and the field. We present new X-ray/UV diagnostics for assessing nuclear activity and compare with earlier results in several different extragalactic environments. We examine global correlations of compact group X-ray luminosity with star formation rate and stellar mass, and compare with similar relations established in other environments. We discuss the unique nature of compact groups in light of these results.

Tuesday, July 10, Talks

Session IV: Simulations of Groups and Clusters

What Simulations Can Tell Us about the Gas Content of Galaxies in Groups and Clusters

Greg Bryan
Columbia University

The gas content of galaxies is observed to depend strongly in environment, a key observation which affects the star formation rate and morphology of these galaxies. These systematic differences in gas content can be due to a variety of effects, including: intrinsic differences in dark matter halos properties, ram pressure stripping from the hot intra-cluster medium, a cut-off of the gas supply ("starvation"), galaxy collisions, as well as gravitational tidal effects. In this talk, we review recent numerical results on all of these mechanisms, demonstrating that simulations can quantify the importance of each effect, and in doing so, shed light on the basic physics of galaxy formation and evolution.

Quenching Star Formation on Cluster Infall

Mike Hudson
U. Waterloo

I will discuss multiple lines of evidence for quenching processes within rich clusters that allow us to disentangle the timescales and physical processes at work: (1) The strong dependence of stellar population ages as a function of cluster-centric radius (2) Evidence of stripping from tidal tails (3) Fading of disks (4) Models based on N-body simulations which tie all of these together.

Satellite Galaxy SFR Evolution in Groups/Clusters

Andrew Wetzel, Jeremy Tinker, Charlie Conroy, Frank van den Bosch
Yale University

Satellite galaxies in groups and clusters are more likely to have quiescent SFRs than galaxies in the field. Using galaxy group/cluster catalogs from SDSS Data Release 7, I examine in detail the SFR distribution of satellite galaxies and its dependence on satellite mass, halo mass, and halo-centric radius. Using a high-resolution cosmological simulation to track satellite orbits, I constrain the mechanisms and timescales of satellite star formation quenching that fit these observed dependencies. Satellite quenching likelihood can be simply parametrized by its time since infall: satellite star formation evolves environmentally unaffected for 2 - 4 Gyr after infall, after which SFR rapidly fades with a e-folding time of < 1 Gyr. More massive satellites quench more rapidly, but the quenching timescales have no dependence on group/cluster mass. Because of the long time delay after infall before quenching occurs, satellites have experienced significant stellar mass growth, nearly identical to that of central galaxies over the same timescale. Finally, I compare these quenching timescales with observed atomic/molecule gas depletion timescales, and I discuss the implications for satellite gas accretion, consumption, and stripping in the dense group/cluster environment.

The Zurich Environmental Study (ZENS) of Galaxies in Groups Along the Cosmic Web

Craig Rudick , Marcella Carollo, Anna Cibinel, Antonio Pipino, Ting Lu
ETH Zurich

ZENS is a survey of nearby (z~0.05) galaxy groups in the mass range ~10¹²-10¹⁴ M_⊙. Using both spectroscopy and deep optical imaging, we have analyzed the structural, stellar population, and star-formation properties of the group galaxies. We determine the dependence of galactic properties, at fixed stellar mass, across a range of environmental indicators - including group halo mass, group-centric radius, large scale structure density, and satellite vs. central galaxies. The most significant effects are seen for satellite galaxies with group-centric distance, where galaxies nearer the group centers are more likely to be quenched, bulge-dominated, and have redder colors (particularly in the disk component) than galaxies in the outskirts. Group halo mass, LSS-density, and the central/satellite dichotomy tend to have smaller, although not always negligible, effects. Environmental effects are more pronounced in galaxies at lower stellar masses. We compare these results to several semi-analytic models of galaxy evolution. We find that the standard recipes tend to predict both an over-abundance of, and overly red colors for, quenched galaxies in groups. Instead, a model in which the star-formation rate of galaxies is tied to the observed cosmic specific star formation evolution more accurately reproduces the numbers and colors of quenched galaxies.

The Galaxy Cluster Concentration-Mass Relation

Elena Rasia
U. Michigan

The concentration-mass relation represents a valuable tool to constrain cosmological parameters such as matter density and sigma_8. In the last few years, samples of optical and X-ray data led to the conclusion that the observed relation has a higher normalization and slope than predicted by dark-matter only simulations. In this work, we explore whether this disagreement is real and, if so, why it exists. To this purpose, we consider 30 clusters simulated by progressively increasing the simulation complexity: (i) dark-matter only, (ii) non-radiative hydrodynamics, (iii) adding cooling, star-formation and feedback by Supernovae, (iv) further adding feedback by AGN. We produced X-ray and optical synthetic catalogues to derive the concentration-mass relation following an observational approach. We find that even if cooling has the effect of steepening the concentration-mass relation with respect to the DM-only simulations, the introduction of AGN makes this difference small (5-10%). A larger variation is expected when reducing the radial range over which density profiles are fitted to a NFW profile. In particular if the external radius is about half R_{500} the slope can double its value. Therefore, observations, suffering from background contamination, are more inclined to detect a steeper c-M relations. Finally, the strong lensing analysis of the synthetic catalogues shows a bias leading to an over-estimate the concentration of small objects.

Session V: Star Formation in Groups and Clusters

Connecting Star Formation and Gas in Nearby Galaxies

Rob Kennicutt
Institute of Astronomy, Cambridge

The past decade has seen an enormous influx of new measurements of the integrated and spatially-resolved star formation properties of galaxies, and the first truly comprehensive sets of matching data on the atomic and molecular gas distributions. Over the same period a new generation of large-area multi-wavelength surveys of the Milky way are allowing us to quantify the patterns and scaling laws in star formation on scales ranging from the disc as a whole down to star-forming molecular cores. This talk will take stock of what we have learned about large-scale star formation from these new datasets, and highlight the key issues and questions that remain to be addressed.

The Role of the Cluster Environment on the Star Formation Cycle of Virgo Galaxies

Luca Cortese
ESO-Garching

Even though it has been known for decades that the properties of galaxies are tightly linked to the environment they inhabit, we still miss a coherent picture about the role played by the cluster environment on galaxy evolution. Until very recently, one of the main problems has been the lack of information about the properties of the different baryonic components (e.g., gas, stars, dust and metals) taking part in the star formation cycle of galaxies and their variation with local density. Now, wide-area multi-wavelength surveys are finally under way, making it possible to investigate separately the effects of the environment on the different constituents of galaxies. In this talk, I will combine new multiwavelength data (e.g., GALEX-UV, SDSS-optical, Arecibo-HI, Herschel-FIR/submm) for the Herschel Reference Survey, a volume-, magnitude-limited sample of nearby galaxies in different environments, to investigate how the star formation cycle changes from high to low density regions. I will explore how the cold gas, dust and metal content and star formation activity of galaxies vary when moving from the field to the center of Virgo and discuss the implications of these results for galaxy evolution models.

A WISE View of the Abell 2199 Supercluster

Ho Seong Hwang, Margaret J. Geller, Antonaldo Diaferio, and Kenneth J. Rines
Smithsonian Astrophysical Observatory

We use Wide-field Infrared Survey Explorer (WISE) data covering the entire region (~130 deg²) of the A2199 supercluster at z = 0.03 to study the mid-infrared (MIR) properties of supercluster galaxies. We identify a `MIR star-forming sequence' in the WISE color (3.4-12 micron) - 12 micron luminosity diagram, consisting of late-type, star-forming galaxies. At a fixed star formation rate, the MIR-detected galaxies tend to be more metal rich and to have higher surface brightness than those without MIR detection. Using these MIR-detected galaxies, we construct the IR luminosity function (LF) and investigate its environmental dependence. Both total IR (TIR) and 12 micron LFs are dominated by late-type, star-forming galaxies. The faint-end slope of the TIR LFs does not change with environment, but the change of faint-end slope in the 12 micron LFs with the environment is significant: there is a steeper faint-end slope in the cluster core than in the cluster outskirts. This steepening results primarily from the increasing contribution of early-type galaxies toward the cluster center. These galaxies are passively evolving, and contain old stellar populations with weak MIR emission from the circumstellar dust around asymptotic giant branch stars.

Gas Accretion, Starvation & Ram Pressure Stripping in the Virgo Cluster

Jeff Kenney
Yale University

Clusters and their outskirts encompass a wide range of environmental conditions. Most of the spiral galaxies with the largest star formation rates in Virgo are HI-rich galaxies with very extended HI distributions, located in the cluster outskirts. We propose that the disks of these galaxies are actively accreting gas, which enhances their SFRs. Of course most cluster galaxies (located closer to the cluster center) are gas-deficient and have reduced SFRs. There is a wealth of evidence that ram pressure stripping is responsible for producing the many cluster galaxies with truncated gas disks. However the origin of anemic galaxies, with extended but low surface brightness disks of gas and star formation, has been less clear. We propose that these disks are starved, with their gas reservoirs at large radii (perhaps like those seen in the gas-rich galaxies, although on average less extreme) removed by either tidal forces or ram pressure stripping, which stops accretion onto the disks.

The Link between Morphological Disturbance and Star Formation History. The Case of A901/902 with STAGES

Carlos Hoyos, Alfonso Aragon-Salamanca, Meghan E. Gray, Chris Wolf
University of Nottingham

One of the expected consequences of gas-rich galaxy mergers is the enhancement of star formation in galaxies. These merger-induced star formation events will be linked with morphological disturbances in the galaxies. In this work, we use the A901/902 multiple cluster system observed by the STAGES collaboration to explore these ideas. This system has been chosen because it contains galaxies of all morphologies in a wide variety of environments. In addition, a vast dataset of high-quality multiwavelength observations are available, including HST/ACS F606W deep images. We use these data to quantify the degree of morphological disturbance of a large sample of galaxies, focusing on the detection of minor mergers. Using a combination of ultraviolet, optical and infrared photometry, we find a very good correlation between our quantitative indicators of morphological disturbance and the galaxies' specific star formation rates and luminosity-averaged ages: disturbed objects exhibit higher specific star formation rated and appear to be younger than relaxed objects of similar masses. We also find that, as expected, early type (E and S0 galaxies) have completed their structural transformation, presenting undisturbed morphologies, old ages and very low star-formation activity. Thus, we conclude that the observed correlation between the galaxies' structural properties and their star formation histories is driven by spiral and irregular systems.

Galaxy transformation in the SuperGroup environment of Abell 1882

Aparajita Sengupta, William Keel
University of Alabama

Since clusters grow by accretion through filaments, and accreted galaxies undergo transformations due to environmental effects, radial locations of these transformations in cluster outskirts can map to a time sequence. In relaxed clusters, several evolutionary mechanisms take effect on similar spatial and temporal scales, making it almost impossible to disentangle different local and global mechanisms. A Supergroup (SG), on the other hand, has shallower Dark Matter potential. Here, the accreting galaxies are subjected to evolutionary mechanisms over larger time and spatial scales. This separates processes that are otherwise superimposed in rich cluster-filament interfaces. We have studied the SG Abell 1882 (z~0.136) using multi-wavelength data from SPITZER, GALEX, XMM, Chandra and SARA. Color-Density Relation is in place or at least in the last stages of being sealed even at this early stage of structure formation. We detect enhanced star formation followed by sudden quenching at the feeding filaments close to Abell 1882. The asymmetry index shows increased disturbed morphology of galaxies due to mergers/interactions even before they enter X-Ray medium of Abell 1882. Hence, significant transformation of filament galaxies occur before reaching X-ray gas or the dense groups. We are gathering further data to separate roles of ram-pressure and tidal effects.

Environmental Imprint on Galaxy Chemical Enrichment
Jorge Iglesias-Páramo, Petropoulou V., Vílchez J.M.

Instituto de Astrofísica de Andalucía (CSIC)

Recent results will be presented on the metal enrichment of low-mass star-forming galaxies in local Universe clusters. The environmental effect on the chemical evolution of these galaxies will be discussed. We have used spectroscopic data from the SDSSIII-DR8 and we have derived the gas-phase O/H and N/O abundances. We have then examined the Mass-Metallicity Relation and the Fundamental Metallicity Relation of this sample of cluster galaxies, and we have found well defined sequences. A flattening of the slope of these sequences has been observed for galaxies located in the core of the two more massive clusters of the sample, suggesting that the effect of the cluster environment depends both on the galaxy mass and the host cluster mass. Based on these results we explore cluster-specific effects (e.g. ram-pressure stripping, pressure confinement etc), predicted by hydrodynamic models, capable of yielding the observed mass-dependent enhancement of metallicity.

How Fossil Are Fossil Groups?

Eric M. Wilcots, Kelley M. Hess, Hanna A. Herbst, Victoria Hartwick
University of Wisconsin

A fossil group is believed to be the merger remnant of a galaxy group whose most massive members have succumbed to dynamical friction leaving behind a single giant elliptical, a faint dwarf galaxy population, and an extended hot gaseous halo. In a sense a fossil group is what is left when the gas runs out and the star formation stops in a galaxy group. We present the results of the first systematic study of the radio continuum properties of fossil groups showing that they remain remarkably active. We also explore the star formation history of fossil groups, showing evidence of a low level of recent star formation. The key to understanding what fossil groups can tell us about the assembly of groups in general lies in looking at their large scale environment to determine the extent to which fossils are still likely to further accrete massive satellites or whether they truly are an end-state of the dynamical evolution of a group and its environment.

Wednesday, July 11, Talks

Session VI: Dwarf Galaxies in Groups and Clusters

Dwarf Galaxies in Nearby Groups and Clusters

Henry Ferguson
Space Telescope Science Institute

Dwarf galaxies in clusters are generally gas poor, and resemble in many ways the dwarf spheroidal population orbiting isolated galaxies. However, the number of dwarfs per giant galaxy in clusters is much larger than the number of dwarf satellites per isolated galaxy. We review progress on understanding the evolution of dwarfs in the group and cluster environment.

Dwarf Galaxies in Groups within 5 Mpc: Resolved Stellar Populations

Lidia Makarova, D. Makarov, M. Koleva, P. Prugniel, et al.
Special Astrophysical Observatory, Russia

Nearby dwarf galaxies are well recognized now as central objects under study for many areas of extragalactic astronomy, such as star formation, galaxy evolution and structure. About 200 nearby galaxies have already been observed with HST/WFPC2 and HST/ACS, either by our collaboration or by others. This outstanding material includes images of galaxies of essentially all types, though dwarfs predominate. The observations offer a unique opportunity to study star formation history directly, from the individual stars resolved in the galaxies. We describe the technique for determining the history of star formation in galaxies based on the photometry of the resolved stars in a galaxy, using a library of synthetic color-magnitude diagrams from theoretical stellar isochrones, taking into account the initial mass function, distance to the galaxy, internal and external absorption, and photometric errors. We consider star formation history measurement results in the nearest galaxy groups. The Local Group are well studied now due to relatively small distances to its members. We can resolved old fainter stars including horizontal branch and red clump stars. Less information can be received on resolved stars in the other two nearest groups: Cen A and M 81. However, HST with new ACS instrument allow us to study oldest red giant population as well. The complex star formation was detected in the galaxies under consideration.

Galaxy Multiplicity in Low-Mass Haloes: the Baryonic Content and Star Formation Properties of Dwarf Galaxy Groups

Ruben Sanchez-Janssen, R. Amorin, A. Lopez-Sanchez, A. Begum
ESO

Dwarf galaxy groups are rare systems, whose existence is controlled by two competing effects: while associations of low-mass haloes are expected in the context of structure formation, galaxy formation within these haloes is an extremely inefficient process. Although their detection is thus extremely challenging, several of these dwarfs groups have been recently identified in the Local Universe by Tully et al. These are bound (but probably unvirialised) associations with total masses of 10¹¹-10¹² M_⊙, characterized by their high M/L and low gas content. Here we present multiwavelength -UV, optical and 21cm- observations of two dwarf galaxy groups recently identified in the SDSS. Surprisingly, these are gas-rich systems dominated by closely-interacting dwarfs that exhibit a great variety of star formation activities. These groups provide unique clues on i) the baryonic (stars and gas) content of low-mass groups; ii) the role of interactions in triggering star formation activity, and iii) the halo occupation distribution at the very end of the halo mass function.

Session VII: Connection to High Redshift Groups and Clusters

High Redshift Groups and Clusters

Michael Balogh
University of Waterloo

Strong constraints on galaxy formation theory and cosmology can be obtained from the redshift evolution of groups and clusters. The availability of deep Xray, SZ and optical data have opened up the redshift range 1<z<2 to regular detection and study of groups with masses as low as 10¹³ M_⊙. In this talk I will review the state-of-the art observations of these systems. I will highlight recent discoveries and focus on new and remaining questions, and future prospects.

Star Formation and Cold Gas in Superclusters at z~0.2

Yara Jaffe, Poggianti, B. M., Verheijen, M., Deshev, B., van Gorkom, J., Yun, M., Cybulski, R., Chung, A., Montero-Castano, M., Morrison, G.
INAF, Osservatorio Astronomico di Padova

We will present first results from the Blind Ultra Deep HI Environmental Survey (BUDHIES) of the Westerbork Synthesis Radio Telescope (WSRT). BUDHIES has already detected cold gas in hundreds of galaxies at z~0.2. We have also recently finished a spectroscopic campaign that has allowed us to characterize the environments of the HI-detected galaxies. Our study is the first where optical properties and gas content are combined at a redshift where evolutionary effects begin to show. In this talk, we will present a study on the environments of star formation and star formation fuel in 2 superclusters at z~0.2. In particular, we will show that the HI gas lives outside of more massive halos, and discuss its fate in cluster environment.

Star Formation in Groups of Galaxies in the COSMOS Survey

Charles Liu, Stephanie Fiorenza (CUNY/CSI)
CUNY College of Staten Island

An X-ray selected sample of 165 groups of galaxies with secure optical counterparts in the redshift range 0 < z < 1 has been assembled from the COSMOS HST Treasury multi-wavelength survey. Within these groups, ~4600 member galaxies have been identified, some by spectroscopy and most by multiband photometry. Both group and member galaxy catalogs (now publicly available at http://irsa.ipac.caltech.edu/data/COSMOS/tables/groups/) are being used in investigations of star formation rates in group environments as a function of group and galaxy mass, density, and redshift, including an examination of what might account for recent observations suggesting that groups with halo masses log(M/M_⊙) ~ 13.5 are particularly efficient at converting baryons into stars.

Central Galaxies in the GEEC Groups

Laura Parker and the Group Environment and Evolution Collaboration
McMaster University

The Group Environment and Evolution Collaboration (GEEC) sample of intermediate redshift groups is a diverse population with a wide range in velocity dispersion, dynamical state and member galaxy properties. The original GEEC groups were identified using optical spectroscopy, but we now have a complementary sample of X-ray selected groups in the GEEC fields. We will present results on the most massive group galaxies (MMGs), including their offsets from the group dynamical centres as well as correlations between MMG properties and group properties such as X-ray brightness and dynamical state. The dynamically young groups in our sample lack a centrally dominant galaxy while the more evolved systems tend to have extensive X-ray emission and a centrally dominant MMG sitting at the centre of the potential. Using the diverse GEEC sample we hope to understand the growth of central galaxies in the group environment and the connection between group properties and their MMGs.

2 Clusters in One Shot: Hα Derived Star Formation in Cluster Candidates at z~0.6 and 0.8 behind MG2016+112

Ulrike Kuchner, B. Ziegler, V. Ivanov, H. Dannerbauer
Institute of Astrophysics, University of Vienna

We present our NIR narrow-band imaging study of SF galaxies in the MG 2016+112 field containing 3 cluster candidates at z~0.6, 0.8 and 1 according to previous work by Soucail et al. 2001 and extended x-ray emission. We probe the 2 lower z clusters with 4 filters using SOFI@NTT with FOV=5'x5' (2x2Mpc) detecting Halpha fluxes down to ~7x10^-17 erg cm^-2 s^-1. Our 2 main aims are to trace the structures of the clusters and to characterize the properties of the galaxies. We utilize Halpha as a calibrator for SF and calculate SFRs with respect to J band continuum. In addition, optical broad-band imaging allows to estimate phot-z, SFHs and stellar masses. We compare our study both to local surveys and other investigations of high-z clusters like EDisCS.

Monday, July 9, Posters

Galaxy Evolution in Cluster Environments at Low-to-Intermediate Redshift

Ryan Cybulski
University of Massachusetts

We will present a technique for characterizing Large-Scale Structure (LSS) in the environments surrounding massive galaxy clusters to probe the effects of environment on galaxy color, morphology, gas and dust content, and star-formation rate (SFR). With many environmental factors potentially driving galaxy evolution between the cluster core and the outskirts, a fair assessment of environmental effects requires wide-area mapping of the surrounding LSS. Our technique uses galaxies selected by photometric redshifts (photo-z), which enables us to examine LSS over an area of ~100 sq. Mpc around our clusters. Since we probe well beyond the virial radius, we are sensitive to galaxy evolution extending to the interface between the cluster and the field. From the photo-z selected galaxies we use a combination of Voronoi Tessellation and Minimal Spanning Tree (MST) analysis to characterize the local surface density of galaxies, and the shape and extent of the LSS traced by those galaxies. We will present our initial results from applying this technique to a sample of galaxy clusters at low-to-intermediate redshifts (0.2 < z < 0.8).

The Gaseous Evolution of Galaxies in Groups

Kelley M. Hess & Eric M. Wilcots
University of Cape Town

We present an analysis of the HI content of galaxy groups as a function of their global environment and their dark matter halo mass. The Arecibo Legacy Fast ALFA survey α.40 data release allows us, for the first time, to study the HI properties of over 700 galaxy groups in the volume of sky common to the Sloan Digital Sky Survey and ALFALFA surveys. We assign HI detections group membership based on an existing SDSS group/cluster catalog and find that only 25% of all HI detected galaxies reside in groups or clusters, in contrast to approximately two-thirds of all optically detected galaxies. Further, we find strong evidence for the gaseous evolution of galaxies as a function of group membership which we use as a proxy for the parent dark matter halo mass of the group. From the spatial distribution of HI detected galaxies, it is evident that the infall of gas rich objects is important to the continuing growth of large scale structure at the present epoch.

Total Neutral Hydrogen Contents of Hickson Compact Groups

G. Lyle Hoffman
Lafayette College

Using a modified version of the Lovedata flux-measuring software, total integrated neutral hydrogen fluxes of the Hickson Compact Groups within the ALFALFA footprint are measured. Most of the HCGs are unresolved by the 3.5' beam and show no evidence for significant emission outside the compact group boundary, but HCG 1 is an exception. Comparison to synthesis array mapping, where available, is made.

A Tidal Dwarf Galaxy in the Interacting Galaxy Group NGC 3166/9?

Karen Lee-Waddell, K. Spekkens, S. Stierwalt, M. P. Haynes, J. Chengalur, P. Chandra, R. Giovanelli
Royal Military College of Canada / Queen's University

We have obtained high-resolution HI observations of the nearby, interacting galaxy group NGC 3166/9 using the Giant Metrewave Radio Telescope (GMRT). Single-dish Arecibo Legacy Fast ALFA survey (ALFALFA) maps of this region show an extended HI envelope around the main group members and reveal several putative dwarf galaxies. Our follow-up GMRT observations are an important probe of the membership, properties and nature of this dwarf galaxy population, which can constrain galaxy formation and evolution in a group environment. A thorough search of the GMRT datacube revealed eight low-mass objects, with gas masses ranging from 4x10⁷ to 3x10⁸ solar masses, for which we computed stellar masses and star formation rates using ancillary SDSS and GALEX data respectively. Most of these detections are likely to be either primordial dwarf irregular galaxies or short-lived tidally formed "knots"; however, one candidate is clearly associated with a prominent tidal tail extending below NGC 3166 and is a probable tidal dwarf galaxy with a dynamical to gas mass ratio close to unity.

The Environmental Dependence of E/S0 Disk (Re)Growth

Amanda Moffett , Sheila Kannappan, Andreas Berlind, Kathleen Eckert, David Stark, David Hendel, Norm Grogin
University of North Carolina at Chapel Hill

Low-mass E/S0 galaxies frequently display extended ultraviolet (XUV) and/or UV-Bright (UV-B) disks, representing newly forming disks with differing radial extents and star formation intensities (Moffett et al. 2012). The ability of such early-type galaxies to switch on and maintain star-forming disks may depend on conditions of the galaxy environment, possibly enabled by local interactions or large-scale "cold flows." In this contribution, we present results from the first study of star-forming E/S0s (including XUVs and UV-Bs) in a statistical sample that allows the construction of a complete picture of local and global environments using smoothed density fields, group finding, and identifications of walls and filaments. In order to examine the reservoirs of gas available to fuel disk (re)growth in these galaxies, we also use public ALFALFA data available for a significant subregion of our catalog to analyze E/S0s with unusually high HI content and relate these excesses to environment.

Do Active Galactic Nuclei Prefer Compact Group Galaxies?

Jubee Sohn (SNU), Myung Gyoon Lee (SNU), Ho Seong Hwang (SAO)
Seoul National Unversity

We present results of a statistical study on nuclear activities of compact group galaxies. What triggers the Active Galactic Nuclei (AGN) is still a puzzling problem. One of the suggested AGN triggering mechanisms is galaxy interaction. Several simulations have shown that gas can be supplied to the center of galaxies during galaxy encounters. In this regard, compact groups of galaxies are an ideal laboratory to study the connection between galaxy interaction and nuclear activities, because of their high densities and low velocity dispersions. To study environmental effects on nuclear activity, we select 59 genuine compact groups using the radial velocities of galaxies in the catalog of SDSS Compact groups (McConnachie et al. 2009). We classify nuclei types of compact group galaxies using the emission line data. We find that 19% of the compact group galaxies are star-forming, 10% are transition objects, and only 7% are AGN. This result is contrary to a much higher AGN fraction ( >40%) in Hickson Compact Groups reported by previous studies. The AGN fraction of genuine compact groups is higher than that of galaxy clusters, but lower than that of field environment. Implications of this result will be discussed.

Feedback in the Local Universe: The Relation between Star Formation and AGN Activity in Typical Elliptical Galaxies

Sravani Vaddi, Chris O'Dea, Stefi Baum
Rochester Institute of Technology

Aim: We address the relation between star formation and AGN activity in a large sample of nearby elliptical galaxies. Feedback from the AGN is believed to be playing an important role in regulating the star formation. Earlier studies of AGN feedback have focussed on large systems, i.e. cooling flow clusters and/or X-ray selected sources and none of the studies looked at the relation between star formation and AGN activity in average elliptical galaxies.
Methods: We are carrying out multi-wavelength study of a sample of 237 elliptical galaxies with the archival observations from Chandra, VLA, Galex, Spitzer and WISE. We obtained the radio power, Galex FUV - 2MASS K color to study AGN activity and star formation. The galaxy samples are unbiased regarding their star formation and radio source properties.
Preliminary results: A color-color diagram shows the presence of bluer galaxies indicating that star formation is present in the sample. There is an upper envelope of radio power that is a steep function of absolute K-band magnitude of the galaxy suggesting that the maximum possible jet power is dependent on the brightness of the galaxy and thus on the mass of the galaxy. Radio power and FUV-K color is uncorrelated suggesting that perhaps fuelling of radio sources is not associated with substantial star formation or perhaps the radio jets have suppressed star formation.

Social Networks and the Structure of Galaxy Groups

Eric M. Wilcots, Ali Bramson
University of Wisconsin

The identification of galaxy groups has always been a challenge and the until the advent of recent large scale surveys groups could only be reliably selected based on the presence of a diffuse hot intergalactic medium. Large spectroscopic surveys changed that utilizing a friends-of-friends technique to identify groups. While FoF has proven effective, it is also quite limited, providing little insight into the substructure of groups or into linkages between groups. In this presentation we explore the possible application of social networking algorithms to characterize groups, their substructure, and their connections across the large scale structure of the Universe. We explore the substructure within individual well-defined groups. In addition, we show that a hierarchical clustering method reveals a wealth of structure on all scales across the Coma-Abell 1367 supercluster and raises questions about what groups really are.

The HI content of the Ursa Major region

Kathrin Wolfinger
Swinburne University of Technology

We investigate the nearby Ursa Major region using the blind HIJASS survey (HI Jodrell All-Sky Survey) to shed light on how environment affects the evolution of nearby gas-rich spiral galaxies. The 480 deg² region includes the Ursa Major cluster and the Canes Venatici groups that connect Ursa Major and the Virgo cluster, covering a heliocentric velocity range of 300-1900 km/s. We compiled a peak-flux limited catalogue containing 159 HI sources; 20 of which are newly detected in HI including 8 candidate galaxies/HI clouds. We study the abundance of interacting systems in different density regions and determine that the majority lies within the Ursa Major cluster. We will further discuss the nature of the Ursa Major cluster by comparing galaxy optical (SDSS) and HI properties. By examining the substructures and dynamics, we probe the hypothesis that Ursa Major is an unevolved and a newly forming cluster and if it will eventually merge with Virgo to form a larger system in the context of hierarchical structure formation.

Tuesday, July 10, Posters

HI Content of Galaxies in the NGC 5846 Group Region

Halley Darling '13, Rebecca Koopmann, ALFALFA Team
Union College

We present HI properties of galaxies in the NGC 5846 group of galaxies, based on analysis of data from the ALFALFA and Sloan Digital Sky Surveys. The Union College team is analyzing a 9x7 degree region surrounding the group as part of the collaborative Undergraduate ALFALFA (Arecibo Legacy Fast ALFA) Team Groups Project. About one-third of NGC 5846 group members are detected in HI compared to 70% of galaxies in the environment. Stacking of ALFALFA spectra for 36 early-type dwarf group members shows no HI gas in these objects. This work has been supported by NSF grants AST-0724918, AST-0725267, and AST-0725380.

The Local Cluster Survey

Rose Finn, A. Earle, A. McCann, G. Rudnick, V. Desai, V., R. Koopmann, K. Rines, M. Balogh, M., B. Poggianti, D. Zaritsky, M. Haynes, P. Jablonka
Siena College

The primary goal of the Local Cluster Survey is to measure the variations in the spatial extent of cold disk gas relative to the stellar disk for approximately 400 low-redshift group and cluster galaxies (z < 0.037) in order to quantify the relative importance of the physical mechanisms that cause galaxies to evolve from blue, actively star-forming galaxies to red, passive galaxies. The sample consists of 9 groups and clusters that span a range of X-ray luminosities, and all have optical photometry and spectroscopy from the SDSS, infrared 24-micron imaging from the Spitzer Space Telescope, and radio data from the ALFALFA survey. The wide areal coverage of these data allows us to track the evolution of disk gas from the dense cluster core to the surrounding field. Here we present a preliminary analysis of the stellar and gas radial profiles for the group and cluster galaxies. We compare with those of the surrounding field galaxies to look for signatures of environmentally-driven gas depletion.

Membership & HI Deficiency of Zwicky Cluster 1400+0949

Steven Goldman & Aileen O'Donoghue
St. Lawrence University

The galaxy cluster Zwicky 1400+0949, also known as MKW 12 and the NGC 5416 group, has been observed in radio as part of the Arecibo Legacy Fast ALFA HI Survey (ALFALFA) and in visible light as part of the Sloan Digital Sky Survey (SDSS). We are investigating this cluster as part of the Undergraduate ALFALFA (UGA) Team Groups project, a collaboration of 11 undergraduate institutions. We have used the IDL program, Group Membership developed by the Skidmore UGA group to identify cluster members (56) and potential members (139) from position and redshift of galaxies in the Arecibo General Catalog (AGC). Of these 195 galaxies, 97 had HI flux measurements from which the observed HI mass could be calculated. All of these were cross-matched with SDSS and 95 had measurements of their isophotal major and minor axes. These were used to calculate the HI deficiency following the method of Toribio et al (2011). Sixty-two of the 97 had Burstein Type and blue major axis measurements in the AGC, allowing use the method of Solanes et al (2001) to calculate their HI deficiency. Thirteen galaxies are HI deficient accord to Solanes et al, and only 6 of those were cluster members. None of the galaxies are HI deficient according to Toribio et al.
These calculations may indicate that Zwicky 1400+0949 is not an independent cluster, but a spur of the Coma Supercluster aligned with our line of sight.

The Herschel View of the Coma Cluster

Scot Hickinbottom, C. Simpson, P. James, E. Ibar, with the assistance of the H-ATLAS team
Astrophysics Research Institute, Liverpool John Moores University, UK

We present the results from a deep Herschel/PACS survey of the Coma Cluster, covering a two square degree region including the core and South West infall region. We combine these with Spitzer and SPIRE data to construct luminosity functions at wavelengths from 24-500μm. As the richest nearby cluster, the Coma Cluster provides an important benchmark in the study of the cluster environment and its effect on galaxy evolution. Clusters at a comparable distance are not as rich, and clusters of a comparable richness are at a greater distance. We therefore compare results with the Virgo Cluster data, produced by the Herschel Virgo Cluster Survey (HeViCS), to investigate the masses of cold dust as a function of Hubble type and thus determine how the richness of the environment impacts on the efficiency of ISM-stripping.

Environmental History of Galaxies in High-Density Regions

Michaela Hirschmann, Gabriella De Lucia, Simone Weinmann, Dave Wilman, Angela Iovino and Olga Cucciati
Osservatorio Astronomico Di Trieste - INAF

We present a detailed analysis of the "environmental history"of galaxies, using galaxy merger trees obtained from applying a semi-analytic model to a large cosmological dark matter simulation. In addition to halo mass, we consider different definitions for calculating the environmental density, which are often used in observational studies. In order to quantify the effect of environmental history and to constrain the time-scale and nature of physical processes that establish the observed trend, we consider observational estimates (SDSS) for the quiescent fraction of galaxies (using sSFR as a tracer) and their variation with density, halo mass and parent halo-centric distance. Comparisons with observational estimates suggest relatively long gas consumption time-scales in satellite galaxies. In the framework of our models, this appears difficult to achieve by using a non-instantaneous stripping of the hot gaseous component associated with galaxies, and points towards a more fundamental problem in modelling the star formation and feedback process in satellite galaxies.

A WISE/GALEX View of Red Sequence Galaxies

Jongwan Ko (Yonsei University Observatory, Korea), Ho Seong Hwang (Smithsonian Astrophysical Observatory, USA), Sang-Hyun Chun, Y.-J. Sohn (Yonsei University, Korea)
Yonsei University Observatory, Korea

We present mid-IR (MIR) and near-UV (NUV) properties of red sequence galaxies defined by optical color-magnitude relation. Despite the common picture of red sequence galaxies in cluster environments as quiescent ones with no star formation, recent NUV/MIR studies of early-type galaxies on the red sequence revealed a wide spread of NUV/MIR colors, implying a variety of NUV/MIR emission. We focus on the properties of the red sequence galaxies with NUV/MIR excess, comparing the properties of post-starburst galaxies to trace how galaxies migrate to the red sequence.

Dynamical Mass Estimates of Galaxy Group AWM3

Nathan Nichols, J. Patterson, C. Weigel, P. Troischt, ALFALFA Team
Hartwick College

The UAT Groups Project: The HI Mass Function for Galaxies in Groups

Mary Crone Odekon, Christopher McGowan '12, Lyle Reed '12, Adina Micula '14, and the Undergraduate ALFALFA Team
Skidmore College

The ALFALFA project is a blind HI survey optimized for detecting small galaxies in the local universe. As part of the Undergraduate ALFALFA Team Groups Project, we are focusing on the HI content of galaxies in groups, which is expected to be a sensitive indicator of galaxy interactions - including mergers, tidal distortions, and triggered star formation. Here we present the composite HI mass function for a sample of twelve groups, where member galaxies are systematically selected using an algorithm we developed for this project.

Orbital Dependence of Quenching Mechanisms

Kyle Oman & Mike Hudson
U. Waterloo

Quenching in clusters is proposed to be tied to a galaxy's orbital history. For instance, a simple model is one where star formation is suppressed after the first pericenter passage. Galaxy orbits are not directly observable, however components of the phase-space coordinates are (projected cluster-centric radius, line of sight velocity). We have constructed a catalog of orbits from the Multidark simulations with the help of the Rockstar and Consistent Trees codes. This catalog is used to relate the observable coordinates of galaxies to the timing of landmark events in orbital history (e.g. first crossing of R_200, pericenter passage, turnaround) with a well-defined statistical confidence. This relation will be used to explore quenching models that depend explicitly on the orbital history of the galaxy to produce predictions that are directly comparable with observations of quenching systems.

HI Deficiency Estimates in ALFALFA Galaxy Groups

Parker Troischt, J. Patterson, N. Nichols, C. Weigel, ALFALFA Team
Hartwick College

The Undergraduate ALFALFA (Arecibo Legacy Fast ALFA) Team Groups Project is a collaborative undertaking of faculty and students at 11 institutions, aimed at investigating properties of galaxy groups surveyed by the ALFALFA blind HI survey. As part of the project, we are examining HI deficiency in galaxy groups and clusters using HI fluxes from ALFALFA and optical data from the Sloan Digital Sky Survey. The HI deficiency calculations reported here are based on comparisons to a sample of 1,624 reference galaxies in low-density environments recently reported by other members of the ALFALFA team. Their sample is taken from the recent α.40 data release and has several advantages over previous HI samples including a large number of high quality detections and homogeneously measured HI fluxes. This sample provides a reliable set of reference galaxies for the HI deficiency calculations performed here. This work has been supported by NSF grants AST-0724918, AST-0725267 and AST-0725380.

The ALFALFA H-alpha Survey

Angela Van Sistine, John Salzer
Indiana University

The ALFALFA H-alpha survey uses a volume-limited sample of HI-selected galaxies from the ALFALFA survey to study star formation in the local universe. When complete, this project will provide narrow-band H-alpha images of ~1400 HI-selected galaxies. ALFLAFA detects galaxies in our survey volume with HI masses smaller than 10⁸ M_⊙, probing well into the flat portion of the HI mass function. Our primary science goal is to produce the best possible measurement of the local star-formation rate density. We also use our data set to study star formation as a function of galaxy environment. In the current presentation, we explore the star-formation properties of a sub-sample of HI-selected galaxies in high density (group and cluster) environments within our volume. With our unique data set, we are able to study star formation in a sample of galaxies selected based on their capability of making stars while remaining unbiased to the optical properties of the galaxies.

Star Formation in the NGC 5846 Group of Galaxies

Lucas Viani '14, Rebecca Koopmann, Halley Darling, '13
Union College

We examine gas and star formation properties of galaxies in the NGC 5846 group. Narrow-band Hα and broadband R images for a sample of galaxies across the group were obtained at the KPNO WIYN 0.9m with MOSAIC and the CTIO 0.9m SMARTS telescopes. Neutral hydrogen data from the Arecibo Legacy Fast ALFA (ALFALFA) survey are used to trace the cold neutral gas content. The amounts and extents of the star formation regions in a subsample of galaxies are compared as a function of cold gas content and position in the group and compared to those of galaxies located in other environments. This work has been supported by NSF grants AST-0724918, AST-0725267, and AST-0725380.

Wednesday, July 11, Posters

The Missing ISM Mass Problem in Local Group Early-type Dwarf Galaxies

Ilse De Looze, M. Baes, J. Fritz, G. Gentile, T.J. Parkin, C.D. Wilson
Universiteit Gent, Sterrenkundig Observatorium

Detailed studies of the star formation history for three early-type dwarf satellite galaxies of the Andromeda galaxy allow to make predictions of the current gas reservoir in those objects. Those theoretical gas masses account for the left-over gas reservoir when assuming a reasonable star formation efficiency of 10% and for the gas mass returned to the ISM by planetary nebulae since the last star formation activity. Surprisingly, those theoretical gas predictions are at least ten times higher than gas masses probed through current HI and CO(1-0) observations. Based on our Herschel dust continuum observations and new direct gas measurements (CO(1-0), CO(3-2), [CII], [OI]) for the brightest early-type dwarf satellite of M31, NGC205, we could confirm the missing ISM mass problem for this galaxy. We furthermore investigate possible explanations for the deficiency in the galaxy's ISM mass. Wrong assumptions in the theoretical models (i.e. a higher star formation efficiency and/or a top-heavy initial mass function) are found incapable of reconciling theoretical predictions and observations. Alternatively, we argue that tidal interactions with M31 and/or efficient supernova feedback have likely expelled a large reservoir of gas and dust from NGC205 during its recent history.

Close Companions to Brightest Cluster Galaxies: Support for Downsizing and Minor Mergers

Louise O. V. Edwards & D. R. Patton
Yale University

Results from a study of close companions to Brightest Cluster Galaxies (BCGs) are presented. These galaxies have been identified for the purpose of quantifying the rate at which BCGs grow via mergers. By exploiting deep photometric data from the CFHTLS, the number of companions per BCG (Nc) with luminosity ratios down to those corresponding to potential minor mergers of 20:1 is found. Both Nc and the average luminosity in companions per galaxy (Lc) rise steeply with luminosity ratio. The same is true for a control sample of other bright, red, cluster galaxies. The trend for BCGs rises more steeply, resulting in a larger number of close companions, suggesting that the BCGs are likely to undergo more mergers compared to otherwise comparable luminous galaxies. Additionally, this sample of BCGs (with redshifts between 0.15-0.39) shows a higher Nc compared to a local sample of luminous red galaxies, suggesting the younger and smaller BCGs are still undergoing hierarchical formation. The level of contamination from unrelated cluster galaxies is modeled using the Millennium Simulations and found to be moderate within projected separations of 50 kpc. But within 30 kpc, 60% of identified companions are real physical companions. The conclusion is that the additional mass in bound merger candidates down to luminosity ratios of 20:1 could be adding as much as 10% to the mass of a typical BCG over 0.5 Gyr at redshifts of z~0.3.

The Impact of Group Dynamics and Substructure on Galaxy Evolution

Annie Hou and the Group Environment and Evolution Collaboration
McMaster University

It is well known that the properties of observed galaxies depend, at least on some part, on the properties of their host environments. We are particularly interested in investigating how the dynamics of the group environment influence galaxy evolution. We study the dynamical state of massive galaxy groups over a wide range of redshifts (0 < z < 1), using the Sloan Digital Sky Survey (SDSS), Group Environment and Evolution Collaboration (GEEC) and higher redshift GEEC2 catalogs. We look for both substructure and non-Gaussian velocity distributions in all of our systems in order to determine the dynamical state. We then use panchromatic data to study the observed galaxy properties (i.e. colour, blue fractions, star formation rates) as function of the dynamical state of their host group. In this work we are probing both the dynamical evolution of groups and the importance of group dynamics on galaxy evolution at a wide range of redshifts.

Study of Groups of Dwarfs in Nearby Universe

Dmitry Makarov, Roman Uklein
Special Astrophysical Observatory, RAS

We analyze the properties of groups consisting of dwarf galaxies only. The sample contains 126 objects, mainly combined in pairs. The most populated group contains six dwarf galaxies. The majority of systems considered reside in the low-density regions and evolve unaffected by massive galaxies. The characteristic sizes and velocity dispersions of groups are 30 kpc and 11 km/s, respectively. They resemble the associations of dwarf galaxies, but are more compact. On the whole, groups and associations form a continuous sequence. Alike the associations, our groups possess high mass-to-luminosity ratios, what is indicative of a large amount of dark matter present in these Systems.

The Interplay of Gas and Stars in Dwarf Galaxies of Very Nearby Groups: the Showcase Example of ESO 347-G017

Ana Monreal-Ibero, Jorge Iglesias-Páramo, J. M. Vilchez, Y. Tsamis, E. Perez-Montero, M. Relaño, J. Walsh
Instituto de Astrofisica de Andalucia-CSIC

One of the key aspects that remains to be fully understood of the theory of formation and evolution of galaxies is the conditions under which the star formation (SF) episodes take place and what are the consequences for their surrounding interstellar medium. In particular, environment on different scales plays an important role, since it can trigger or quench this SF. Dwarf galaxies are particularly sensitive to this environment since they are relatively fragile systems in comparison to massive spirals. Thus they constitute suitable laboratories for the study of the connection of gas and stars in the SF cycle. To this purpose, unbiased 2D high quality spectroscopic observations able to resolve the different elements (i.e. gas, star, dust) playing a role in this process are needed. We are carrying our a program that intend to study in a homogeneous (i.e. same instrumental set-up) and detailed (linear scale~20 pc/") manner the stellar populations as well as the physical and chemical conditions of the ionized gas of a sample of dwarf Irregulars covering a range in environment, morphology and range of star formation using the VIMOS Integral Field Unit. Here, I intend to present a preliminary view of the data obtained so far.

Variations in the activities traced in mid- and far-IR and HI content in and around z=0.2 clusters

Min Yun, Cybulski, R., Jaffe, Y., Verheijen, M., Chung, A., Deshev, B., Montero-Castano, M., Morrison, G., Poggianti, B. M., van Gorkom, J., Dwarakanath, K.S.
University of Massachusetts

To address the environmental factors responsible for transforming star-forming spirals into quiescent early-types, we have carried out an unprecedented study of the atomic gas, dust, stellar populations, morphology, and star-formation activities of two galaxy clusters at z=0.2 and their surrounding large scale structure (Verheijen et al. 2010, Deshev et al. 2012, Jaffe et al. at this conference). This survey covering an area of 1 X 1 deg² (~100 Mpc²) around the two clusters Abell 963 (z=0.206; extremely rich, X-ray luminous, and dynamically relaxed) and Abell 2192 (z=0.188; less massive, less X-ray luminous, and has significantly greater sub-structure) with a 4σ detection threshold of 2x10⁹ M_⊙ (comparable to the gas mass of the LMC) offers a sensitive probe of the effects of galaxy transformation (ram-pressure stripping, starvation, harassment, and mergers) which act on galaxies in different cluster environments. In this paper, we will present the mid- and far-IR properties of cluster galaxies traced by the Herschel and Spitzer data and compare them with those of the galaxies in the foreground and background regions. In particular, we will discuss the changes in IR properties as a function of local galaxy density and the intrinsic or environmentally induced variations in the cold gas (HI) content.

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