The Physics of Groups
and Galaxy Properties therein


IAP, Paris, 12 to 15 December 2016


Participants and abstracts
AGUERRI J. Alfonso L.Oral
Observational properties of Fossil groups
Fossil groups are those galaxy aggregations dominated by a central early-type galaxy. These large and massive galaxies are the product of an earlier formation through mergers of L* galaxy at high redshift. The Fossil Group Origins (FOGO) project have studied the properties of this type of systems over a large sample of objects and through the electromagnetism spectrum. In this talk I will summarize the main results of the project. In particular, I will show the results related with the properties of the central galaxies, the luminosity function of these systems, their X-ray scaling relations and the presence of galaxy substructure.

ALATALO KatherineOral
Star formation and molecular gas in shocked compact group galaxies
Understanding the evolution of galaxies from the star-forming blue cloud to the quiescent red sequence has been revolutionized by observations taken with millimeter interferometers, allowing astronomers to probe both star-forming molecular gas in a large set of galaxies with unprecedented sensitivity. Recent CARMA observations of of H2-bright Hickson Compact Groups of galaxies (HCGs) have shown that these turbulent systems show suppression of star formation. I will discuss how shocks influence the interplay between galaxy metamorphosis and star formation in the special environment of a compact group, which serves as an idea laboratory for understanding the complex relationship between a transitioning galaxy, and its star-forming fuel.

AMRAM PhilippePoster
Witnessing gas mixing in a nearby compact group of galaxies
We will describe an ideal nearby group (z=0.0137) in which gas flows could be directly detected, HCG 31, the most compact groups of the Hickson’s catalogue. This group consists of low-mass, low-metallicity, intense star forming and clumpy galaxies (similar to the Large Magellanic Cloud) and displays the signature of strongly interacting galaxies in process of merging: low angular separation and velocity dispersion, tidal tails, tidal dwarf galaxies candidates, irregular morphology, complex kinematic, embedded in a large HI reservoir. At least two galaxy members are merging in a bound orbit with almost parallel spin axes in a prograde encounter and the two nuclei will coalesce. Wolf-Rayet spectral features indicate a very recent episode of star formation (<10 Myr) that might be responsible for local nitrogen enrichment near the center while the metallicity decreases in the surrounding of the nuclei between which gas flows are suspected to be observed. We will present new data coming from VLT/MUSE combined to Gemini/GMOS and Fabry-Perot data. This nearby system is especially important because it involves the merging of low-mass and clumpy galaxies, making it an important system for comparison with high-redshift galaxies.

ANN HongBaePoster
Environment Dependence of Dwarf Galaxy Morphology
We present the environment dependence of the dwarf galaxy morphology using the morphological types determined by visual inspection of the SDSS color images. We distinguish the sub-types of dwarf elliptical-like galaxies such as dSph, dE, dEbc, Eblue, and dS0 along with the dwarf irregular galaxies (dI). The morphology of dEbc galaxies is similar to dE galaxies except for the central blue color whereas that of dEblue galaxies is similar to HII region-like BCDs with less blue colors. The dSph galaxies show fainter surface brightness and shallower surface brightness gradient than dE galaxies. The general trend of environment dependence of dwarf galaxy morphology is similar to that of giant galaxies. That is, galaxies with star forming activities are likely to be located in the dense regions while those with no star formation are preferentially located in the under-dense regions. However, the majority of dwarf galaxies are members of groups, mostly as satellites of central galaxies, it is of interest to see whether the dwarf galaxy morphology depends on the environment of the groups or satellite systems they belong to. In particular, we explore the dependence of galactic conformity in group environment.

Probing the state of shocked gas in compact group galaxies
Following the detection of large amounts of shocked molecular hydrogen in Stephan’s Quintet, we have been investigating a sample of galaxies in other Hickson Compact Groups which were found to share some similar properties in term of molecular hydrogen excitation. I will present follow-up optical IFU (VIRUS-P) and Herschel far-IR (PACS) spatially-resolved spectroscopy of gas in a small sample of these systems. The study explores the excitation properties, kinematics and far-IR cooling channels of the diffuse neutral and ionized gas. We also compare these properties with that of molecular gas in the same systems obtained with spatially-resolved CARMA CO 1-0 data on a similar scale. The results address the question of what is the source of excitation of the warm molecular gas, and how the existence of such gas may influence the apparent rapid evolution of some compact group galaxies from the blue cloud to the red sequence.

BABUL ArifOral
The Interplay between Stellar and AGN Feedback in Galaxy Groups
The thermal and chemical properties of the intragroup medium provide important constraints on the feedback processes associated the formation and evolution of massive group central galaxies. I will discuss these constraints and their implications. I will demonstrate that AGN feedback is not necessary in order to understand the observed X-ray and intragroup medium properties of galaxy groups. However, AGN feedback is necessary to account for the observed properties of the group central galaxies. These two assertions jointly provide important insights into the nature of AGN feedback in galaxy groups. Specifically, I will show that gentle ``maintenance-mode" AGN feedback, as has been suggested to occur in galaxy clusters, is insufficient to bring the stellar and baryonic fractions into agreement with the observations. AGNs must be powerful enough to expel gas beyond R200. I will also touch on whether an AGN powered by Bondi accretion is capable of driving outflows powerful enough to do this, and whether there is a viable alternative to Bondi accretion.

Cool gas in the Intra-Group Medium
I present new MUSE observations of galaxy groups probed by background quasars. The MUSE observations offer a wealth of data on the group members, allowing a comprehensive study of the galaxy properties including star-formation rates, H-alpha/OII gas dynamics and stellar dynamics. At the same time, high-resolution quasar spectra trace cool gas absorption components at the group redshifts, showing Ca II, Mg I, Mg II and Fe II species. By exploring the group galaxy properties, I present an analysis on the likely origin of the absorbing gas, evaluating the possibility of: (a) a co-rotating gas halo associated with a single galaxy within the group; (b) outflowing material from a single group member powered by recent star-formation; and (c) cool dense gas embedded within the intra-group medium.

BITSAKIS TheodorosOral
Star formation in compact groups
Star formation is one of the most important properties to understand the formation and evolution of galaxies. Compact groups have been suggested as one of the most interesting environment to study this evolution since they occupy a unique position in the range of galaxy environments. In this review talk, I will briefly describe the recent developments in the study of star formation in compact groups of galaxies. Modern technologies allowed us not only to study the dust obscured star formation activity, but also to identify the possible mechanisms driving its evolution.

BIVIANO AndreaOral
Reconciling kinematic, lensing, and X-ray mass estimates of a poor cluster of galaxies
Abell 315 is a z=0.174 poor cluster, which has been flagged as X-ray underluminous from the comparison of its X-ray luminosity and lensing and kinematic mass estimates. Thanks to a new spectroscopic data-set we are now able to detect a bimodality in the cluster velocity distribution and lower its kinematic mass estimate to log(M200/Msun)=13.9. The cluster X-ray luminosity is no longer anomalous for this low mass. What is anomalous is the cluster very low mass concentration (c200<1). We speculate this may result from a collision between the two substructures that create the bimodality in the velocity distribution of the cluster galaxies.When this low value of c200 is taken into account, also the mass estimate that we deduce from weak lensing is reconciled with the low X-ray luminosity of the poor cluster.

BOURNE MartinOral
Simulation of AGN jet feedback
Radio jets, launched by accretion onto supermassive black holes (SMBHs), and the large X-ray cavities they inflate are often observed in galaxy groups and clusters. Such cavities are filled with highly energetic, relativistic plasma and are expected to play an important role in regulating cooling within the intra-cluster medium (ICM). This in turn can play a central role in determining the host galaxy and group/cluster properties by regulating star formation and black hole growth as well as providing a mechanism through which metals can be redistributed within the ICM. Vast improvements both in observations and computer simulations have helped to shed light upon the mechanisms at play in the jet-cavity-ICM interaction and how this impacts group and cluster properties, however there are still a number of open questions. I will present the results of a set of simulations in which a new jet feedback scheme has been implemented into the moving mesh code AREPO. We have performed comparisons of jet injection techniques in order to understand their impact on jet evolution. Further the simulations investigate the interplay between heating and cooling within the ICM, how jet inflated cavities evolve, the roles of jet driven and large scale turbulence and the mechanisms though which cavities heat the ICM.

BOWER RichardOral
The dark nemesis of galaxy formation
Galaxies fall into two clearly distinct types: `blue-sequence' galaxies that are rapidly forming young stars, and `red-sequence' galaxies in which star formation has almost completely ceased. Most galaxies more massive than $3\times10^{10}\Msol$ follow the red-sequence while less massive central galaxies lie on the blue sequence. We show that these sequences are created by a competition between star formation-driven outflows and gas accretion on to the supermassive black hole at the galaxy's center. We develop a simple analytic model for this interaction. In galaxies less massive than $3\times10^{10}\Msol$ young stars and supernovae drive a buoyant outflow that balances the rate of gas inflow. This prevents high gas densities building up in the central regions. More massive galaxies, however, are surrounded by a hot corona. We argue that above a halo mass of $\sim 10^{12}\Msol$, the supernova-driven outflow is no longer buoyant and star formation is unable to prevent the build up of gas in the central regions. This triggers a strongly non-linear response from the black hole. Its accretion rate rises rapidly, heating the galaxy's corona, disrupting the incoming supply of cool gas and starving the galaxy of the fuel for star formation. The host galaxy makes a transition to the red sequence, and further growth predominantly occurs through galaxy mergers. We show that the analytic model provides a good description of galaxy evolution in the EAGLE hydrodynamic simulations, and demonstrate that, so long as star formation-driven outflows are present, the transition mass scale is almost independent of subgrid parameter choice.

COMBES FrancoiseOral
Molecular gas in tidal and ram-pressure stripped tails
Group and cluster environment can perturb considerably galaxies and transform their morphology in very short time-scales. We will present evidence of gas removal, where not only the external atomic gas is involved, but also inner molecular gas. Star formation still occurs in the tails, however with a much lower efficiency. The survival of dense clouds in harsh environments will be discussed.

The impact of cosmological simulation in understanding the evolution fossil galaxy groups
It has been a long-standing question as if fossil galaxy groups are simply a transient phase in the life of galaxy groups or if they are a distinct class of objects that have experienced a different formation history, compared to ordinary groups. In this review talk, I will briefly explain developments made during the last decade in modelling the formation and evolution of structures in the Universe, based on hydrodynamic simulations of dark matter particles as well as semi-analytic models of galaxy formation, and their contribution in advancing our knowledge of the evolution of fossil galaxy groups.

DUBOIS YohanOral
Morphological transformations and AGN feedback
Active galactic nuclei (AGN) is one important physical ingredient to get realistic massive galaxies, and overcome the cooling catastrophe. With the Horizon-AGN simulation, one of the state-of-the-art cosmological hydrodynamical simulations, I will show how AGN feedback suppresses star formation, while it allows for the rapid morphological transformation of galaxies by means of mergers. I will detail how those mergers transform centrifugally-supported discs into dispersion-supported ellipticals, and change the size of those objects.

DURRET Florence Oral
Groups and clusters in the 3XMM-Stripe 82 zone
We have searched for groups and clusters in all the XMM-Newton observations covering the SDSS Stripe82 region and found 104 objects, among which 54 with confirmed redshifts between 0 and 1. We will present their X-ray (X-ray gas temperature and mass) and optical (luminosity functions, morphological distribution) properties.

EPINAT BenoîtPoster
Kinematics of group galaxies at z~0.7
Galaxies are affected by both internal and external influences and disentangling these effects is crucial for a proper understanding of galaxy evolution. A range of studies have found that the quenching of star formation in galaxies depends mainly on the mass of the galaxy, and, particularly in the case of satellite galaxies, on the location of the galaxies in the halo. Environment quenching is though to be prominent at z < 0.5 (Peng et al. 2010), whereas evolution is dominated by mass effect at high redshift. However the turnover between both effects is not yet well constrained. Whereas large surveys have been recently built in order to explore star-forming galaxy kinematics at z>0.5 using integral field spectroscopy (mainly SINFONI, KMOS and MUSE), none of them are yet able to study with large statistics the impact of both small- and large-scale environment, due to their selection and to the small field of view of instruments used. We are currently conducting an observing program in frame of the MUSE-GTO focused several dense intermediate redshift groups (z~0.7). At these redshifts, the typical size of groups fits the large monolithic MUSE field of view, and the fraction of star-forming galaxies in groups is still quite large. Observing this population may give insights about the processes responsible for star formation quenching in groups. I will present the dataset and a preliminary analysis focused on the galaxy spatially resolved kinematics in those groups. Kinematics will be compared to that of more isolated galaxies.

EVRARD AugustOral
Extending a Galaxy Cluster Population Model to Group-scale Masses
A model in which mean observable features scale as power-laws in total mass, with the population displaying log-normal covariance about these mean trends, is a core assumption in predicting counts of groups and clusters of galaxies for cosmological studies. In this talk, I will review the evidence supporting this model for clusters at the most massive end of the halo hierarchy, and speculate about potential (apparent?) deviations at the lower mass scales appropriate for galaxy groups.

Galaxy groups in ultradeep X-ray surveys
Advent of Chandra and XMM surveys of CDFS, AEGIS, COSMOS, CDFN has opened doors to studies of galaxy groups at high redshift. X-ray selection has enabled unbiased studies of properties of group galaxies, which I will summarize in my talk.

FORMAN WilliamOral
Feedback on group scales - the M87 environment
M87 provides a unique view of the feedback interaction between a supermasive black hole and its environment on scales of a few tens of kpc, appropriate to groups. We combine results from a deep Chandra observation with a simple shock model to derive the properties of the outburst that occurred about 12 yr ago. By modeling the outburst, we are able to partition the SMBH outburst energy between shocks and buoyant relativistic plasma bubbles and show that energy in buoyant bubbles dominates that in shocks. With a multi-wavelength - radio and X-ray - view of M87, we chronicle outbursts over the past ~100 Myr. Finally, we discuss how gas velocity fluctuations, inferred from the observed X-ray surface brightness fluctuations, are produced by the buoyant bubbles and how the resulting turbulent heating can offset radiative cooling. We discuss the implications of M87's outburst history for other group-scale systems.

FOSSATI MatteoOral
Witnessing the onset of environmental quenching at z~2. Results and implications the 3D-HST survey
During the last decade observations of galaxies across cosmic times coupled with cosmological simulations have provided an increasingly clear description of galaxy evolution. In particular we have a fairly detailed phenomenological picture of how galaxies transition from star forming to passive (or quenched) as a function of their internal properties (e.g. stellar mass) and the external environment (e.g. local density). By exploiting the highly complete coverage of grism and spectroscopic redshifts from the 3D-HST survey, we derive the local environment for a deep and complete sample of galaxies in the five 3D-HST deep fields at 0.5 < z < 2.5. A robust definition of environment also requires accurate calibrations obtained using the most up to date semi-analytic model derived from the Millennium simulation. By combining observational data and models we have devised a robust statistical framework within which we link observables to physical quantities (e.g. halo mass and central/satellite status). In this talk I will present our latest results on the environmental quenching of satellites up to z ~ 2.5 in the range of haloes commonly included in our sample Mhalo < 10^14. We find evidences that the quenching timescales for satellites are almost independent on halo mass but have a significant stellar mass dependence. In contrast to local observations we found that for low mass galaxies at z>1 this timescale approaches the Hubble time. I will discuss the physical motivation of these results in terms of quenching mechanisms and gas content of the satellites at the epoch of infall.

Element abundance ratios and star formation quenching in central and satellite galaxies
The properties of galaxies stellar populations are the signposts of how environment affects the galaxy's star formation history. Differences in the stellar populations of central galaxies (the most massive galaxy in a halo) and those of equally massive satellite galaxies have indeed been detected in our previous works (Pasquali et al 2010,2012). Below 10^10.5Msun, satellite galaxies are found to be older and more metal-rich than centrals, with the difference increasing with satellite's group halo mass. We investigate whether such differences are also reflected in the degree of alpha/Fe enhancement, associated to different star formation timescales. We perform a comparative analysis of the alpha/Fe abundance ratio (in addition to mean age and metallicity) of isolated centrals, centrals in groups/clusters and satellite galaxies as a function of the galaxy stellar mass, host halo mass (covering both group and cluster scales) and accretion epoch (as indicated by the location in Delta(V)/sigma - R/R200 phase space). Extending our recent work (La Barbera et al 2014) we include all galaxy types and characterise galaxies in terms of their star formation rate. The negligible differences between centrals and satellites in alpha/Fe abundance ratio at fixed mass pose constraints on the epoch and duration of star formation quenching associated to the accretion of a galaxy onto a group environment.

GAVAZZI RaphaëlPoster
The truncation of Sub-Halos in the AMALGAM cluster sample from weak lensing
Using a large sample of about 120 clusters of galaxies gathered from CFHT and Subaru archival imaging data, we are able to measure the overall mass distribution in clusters with weak lensing signal. The statistical power of the sample, coupled with strong lensing constraints present on small scales for a subset of clusters, allows us to constrain the radial mass distribution of cluster members and test the amount of tidal truncation galaxies can undergo when orbiting in dense cluster environment as compared to the field. We also propose to revisit the multipolar moments of the Dark Matter and galaxy distributions to get new insights on the bias of this latter.

GROOTES MeiertPoster
Gas-Fuelling in the Group Environment - The Star Formation Activity Cycle of Satellite Spiral Galaxies
The accretion of gas onto galaxies (gas-fuelling) is a crucial yet still poorly constrained element determining the star formation history and evolution of a galaxy - particularly in the important environment of galaxy groups. We address this using the Galaxy And Mass Assembly (GAMA) spectroscopic and multi-wavelength survey -- using novel, purpose built techniques of morphological classification and radiative transfer modeling of large samples of galaxies to study the impact of the group environment on the gas-fuelling and current star-formation of non-interacting local universe (z < 0.13) spiral galaxies. From this analysis we find that, contrary to the standard paradigm, gas-fuelling of satellite spiral galaxies is on-going, with a rate similar to that found for non-grouped galaxies and almost independent of group halo mass. In fact, modeling indicates that the star formation history of these satellite spirals may be well described by a cycle of quenching and resuscitation of star formation during their satellite lifetime. Furthermore we explore the implications of our findings for the well known empirical relations of the group environment, e.g. the color density and empirically quantify the group-wide impact of AGN on the star-formation of galaxies in galaxy groups.

The multiphase turbulent intergalactic medium of Stephan's Quintet
I will review what we know about the IGM of Stephan's Quintet from multi-wavelength observations, and consequences about the role of galaxy interaction in driving turbulence and regulating star formation.

Galaxy Clustering and the galaxy-halo connection in the Deep Multi-Wavelength Surveys
Deep wide-field surveys are an important probe of the role of environment from the peak of star formation to today, with it increasingly becoming crucial to take advantage of surveys at multiple wavelengths to get the most from the data. The VISTA Deep Extragalactic Observations (VIDEO) Survey is a key such survey for understanding galaxies on a cosmological scale and probing the 'epoch of activity', when galaxies virialised within their dark matter halos and the majority of star-formation and AGN behaviour occurred. Observing in Z, Y, J, H,K over 12 sq degrees (with fields chosen for the availability of multiband data) and up to z~4, VIDEOs depth and breadth allows both large-scale structure as well as evolution inside individual dark matter halos to be probed up to very early times. We present a series of results from using a clustering analysis to investigate the connection between the galaxies and the host dark matter halo in the first data release of VIDEO. We use the survey to study a variety of galaxy evolutionary processes and environmental effects up to z$\sim$1 and beyond using a halo occupation distribution (HOD) methodology, allowing us to track how stellar mass builds up in different halos over cosmic time. Furthermore we explore how quenching mechanisms can be introduced into the HOD formalism to track exactly where and when quenching occurs within a group environment.

HUDSON MichaelOral
The quenching of star formation and morphological transformations in groups and clusters: when and where
We discuss a novel method of determining when and where the quenching of star formation occurs in groups and clusters. The method is based on the comparison of N-body subhalo orbit libraries with data in projected phase space. We will show applications of this method to the quenching star formation rates and morphological transformation for galaxies in the SDSS.

JONES ChristineOral
Characterizing the X-ray Emission of Galaxies in Groups
Using Chandra observations, we have measured the gas mass and total mass of the halos surrounding ~100 central galaxies in poor groups, as well as their nuclear X-ray emission. We will compare these results for central group galaxies to galaxies at the centers of rich clusters.

JUNEAU StephanieOral
Extremely Obscured AGN in a Nearby Galaxy Group
I will present new results on the link between the most extreme X-ray obscuration of active galactic nuclei (AGN) and host galaxy properties. In particular, I will introduce a case study of a nearby Compton-Thick AGN residing in a galaxy group with a wealth of information to which we added integral field spectroscopy from VLT/MUSE to probe ionized gas and dust structures. These new data reveal interesting signatures over a range of physical scales, and, intriguingly, a possible link between nuclear obscuration and interactions with group members. Overall, our multi-wavelength results call into question the traditional AGN unified model, according to which extreme X-ray obscuration takes place at very small scales within the torus independently from the host galaxy’s state, and yet also do not fulfill the alternative major merger scenario.

KAFLE PrajwalPoster
Mass segregation in galaxy groups
Presence or the strength of satellite mass segregation in galaxy groups has remained a contentious issue. We present results from our recent finding on this topic using all the three types of galaxy group data in our disposal i.e. Galaxy and Mass Assembly (GAMA) observation and GALFORM semi-analytic and EAGLE cosmological hydrodynamical simulations of galaxy formation and evolution. We investigate the redshift range of z<0.32 and the log halo mass range of 12-14.5 dex in Msun. Furthermore, we discuss why and how our result differs from the earlier findings using SDSS group catalogue of Yang et al. 2007. In the end we discuss physical implication of our result in understanding the overall evolution history of galaxy groups.

Fossil galaxy groups; Halo and all therein
Fossil galaxy groups are famed for their symmetric X-ray emitting IGM distribution and optical dominance of their brightest galaxy, arguably the consequence of their early formation epoch. The dynamical relaxation associated with fossil groups makes them simple laboratories, as compared to young/evolving galaxy groups, to study IGM, AGN, their galaxies and halo properties. I will review multi-wavelength studies of fossil groups since their introduction by Ponman et al in 1994. I will provide evidences in support of the early formation epoch and discuss counter arguments.

KRALJIC KatarinaPoster
Environmental effects in at z<0.2 from the GAMA survey
We investigate the properties of central and satellite galaxies in groups in the redshift range 0.02 < z < 0.2 using the spectroscopic survey Galaxy and Mass Assembly. We explore the red fraction, quenching efficiency and mean star formation activity of satellite galaxies as a function of their central’s stellar mass, color and local density. We find that, at fixed stellar mass, both the red fraction and the mean color (or sSFR) of satellite galaxies depend strongly on their central’s color and mass. Our findings confirm earlier results on so-called “galactic conformity” showing that the red fraction of satellites is higher in groups whose central is red than in those whose central is blue. We show that this concept of conformity extends to a continuous correlation between the mean star formation activity of satellites and that of their central, with small mass satellites being the most sensitive to this environmental effect. Surprisingly, quiescent satellites also tend to redden along with their central. This continuous coevolution of the properties of satellites and centrals reinforces the idea of group quenching rather than satellite quenching. This group quenching can be best related to halo mass assuming a central color dependent stellar-to-halo mass ratio, suggesting that assembly bias, which accounts for different accretion histories of halos, is at play.

LAIGLE ClotildeOral
Galaxy properties within filaments using COSMOS2015
The cosmic web is the large-scale environment in which galaxies form and evolve. We use the photometric COSMOS2015 catalog on COSMOS field to investigate the physical link between galaxy properties and their anisotropic environment. The cosmic web is reconstructed from projected two-dimensional slices. The analysis of a virtual photometric catalog produced from the Horizon-AGN simulation tuned to have the same limitations as COSMOS2015 allows us to assess the reliability of the reconstruction. We then analyze galactic properties (mass, sSFR, metallicities, age, ...) as a function of their position within their anisotropic environment (e.g. distance to the filament, distance to clusters node along filaments).

LE BRUN AmandineOral
Scatter and evolution of hot gas properties and of dark matter profiles of the most massive galaxy clusters since z=1
We present an investigation of the scatter and evolution of the hot gas properties (as probed by X-ray and Sunyaev-Zel’dovich observations) of galaxy groups and clusters as a function of the important non-gravitational physics of galaxy formation using the cosmo-OWLS suite of cosmological hydrodynamical simulations. We have previously shown (Le Brun et al. 2014, McCarthy et al. 2014) that a subset of our simulations which includes efficient AGN feedback reproduces a very wide range of properties of the local group/cluster population.  We find that the predictions of the self-similar model break down when efficient feedback is included, for both mass slope and evolution; that the log-normal scatter varies only mildly with mass and non-gravitational physics but displays a relatively strong redshift dependence (it tends to decrease with increasing redshift), and that X-ray temperature is the best overall mass proxy while X-ray luminosity is the poorest. We will also present the first results from a simulation campaign aimed at producing large cosmological simulations that are 1 Gpc/h on a side and have a medium mass and spatial resolution. They are being complemented with very-high resolution zoom simulations which are progressively including the non-gravitational physics of galaxy formation such as star formation, supernova and AGN feedback. The simulations are produced using the AMR code RAMSES. They are being tailor-made to investigate the evolution of the dark matter profiles of the most massive galaxy clusters since z=1.

LILLY SimonOral
Galaxy conformity on small and large scales
I will discuss the phenomenon of "galactic conformity" both within groups and on super-halo scales, looking at both the observational data and the interpretation.

Supression of star formation in transitioning galaxies in Hickson Compact
Galaxies in Hickson Compact Groups (HCGs) live in an environment where galaxy interactions play an important role. Mid-infrared colours from the Spitzer and WISE are able to classify galaxies in blue, star-forming and red, quiescient objects. Galaxies with intermediate colour, populating the so-called “canyon” region are rare. I will present an analysis of the molecular gas and SF properties of a sample of 138 galaxies in HCG with CO data. We find that the star formation efficiency and also the molecular-to-stellar mass is lower in canyon compared to blue galaxies, indicating that a loss of molecular gas and suppression SF is crucial in the morphological transition.

MAMON GaryOral
Measuring group environments
To be written

MAYER LucioOral
Satellites in groups: resolved environmental processes
Galaxy satellites evolve dramatically in groups, undergoing mass loss and quenching of star formation under a variety of mechanisms, from tidal stirring to ram pessure stripping. Understanding quantitatively the action of such processes in groups is crucial to understand galaxy evolution in dense environments. This includes also galaxy clusters since there is evidence that "pre-processing" of galaxies in groups might be a crucial stage even for cluster galaxies. Modeling accurately environmental processes has been possible only with detailed local simulations until recently, without the full cosmological context. I discuss the results of two new sets of simulations, the ErisMOD and GigaERIS simulations, which finally can capture such processes in a fully cosmological context owing to exquisite resolution, of order a few pc spatial and a few hundred solar masses in the baryonic component and a proper treatment of two-fluid interfaces. A highlight result of these new studies is the strong dependence of the mass and morphology of satellites on their dark matter halo density profile. The "too-big-to-fail" problem is indeed shown to be solved naturally for tidally stirred satellites having an almost "cored" inner density profile resulting from the response to supernovae winds. The complex interplay between ram pressure and the cosmic UV background is also shown to be the key to understand the shape of the star formation history of dwarf galaxy satellites in the Local Group.

The hot gas properties of galaxy groups: a theoretical perspective
I will review our current theoretical understanding of the hot gas properties of galaxy groups, with a focus on what cosmological hydrodynamical simulations have taught us. I will compare and contrast groups with lower-mass galactic systems and higher-mass clusters of galaxies.

MCGEE SeanOral
The evolution of satellite galaxy quenching

MERNIER FrancoisPoster
Radial abundance profiles in the hot intra-group (and -cluster) medium
The hot intra-group medium (IGrM) is rich in metals, which are continuously synthesized by Type Ia (SNIa) and core-collapse (SNcc) supernovae within cluster galaxies since the major cosmic epoch of star formation (z ~ 2-3). After their diffusion in the IGrM, these highly ionized metals are detectable in X-ray spectra via their K-shell (and/or L-shell) emission lines. Since some specific elements (e.g. O and Mg) are mainly produced by SNcc, while some others (e.g. Ar, Ca and Fe) are preferably synthesized in SNIa, measuring their abundances in the IGrM helps to constrain the SNIa (/SNcc) fraction contributing to the intra-group enrichment. More specifically, deriving the radial distribution of metals in the IGrM can provide crucial hints to better understand the chemical history of galaxy groups. Here, we present XMM-Newton EPIC observations of 44 nearby (z<0.1) cool-core galaxy clusters, groups, and elliptical galaxies (the CHEERS catalog). In particular, we focus on the average radial abundance profiles of several key-elements (including O, Mg, Si, S, Ar, Ca and Fe). In addition to comparing our results with previous measurements and simulations, we devote special attention to all the systematic uncertainties that might affect our measurements.

OLD LyndsayOral
Using galaxies to measure the masses of galaxy groups and clusters - Where are we today?
While a variety of techniques exist to detect galaxy groups and clusters, their masses cannot be directly measured, but only indirectly inferred from observed properties that correlate with mass. While galaxies contribute only ~1-4% to the total mass budget of a these systems, they trace the gravitational potential well, providing an inexpensive mass proxy. To accurately characterize groups and clusters, and maximise their constraining power for upcoming wide-field surveys (Dark Energy Survey, eROSITA and Euclid etc.), it is essential to examine the level of scatter and systematic bias associated with these commonly-used mass estimation techniques. In this talk, I will present results on the outcome of an extensive blind study, The Galaxy Cluster Mass Reconstruction Project, which was created in order to ascertain how accurately we can measure the masses of groups and clusters using techniques that rely upon the positions, velocities, colours and magnitudes of galaxies. I will focus specifically on the group/low-mass cluster results, describing the impact of various member galaxy selection procedures on the final mass estimate. I will also present the latest results on whether the presence of significant dynamical substructure in groups and clusters impacts mass estimation, indicating where these ‘unrelaxed’ systems should be used for cosmology studies. Finally, I will address the implications of the magnitude of scatter in the recovered masses for future surveys relying on high-mass groups and cluster masses.

The Complete Local-Volume Groups Sample: X-ray observations of optically selected groups
Galaxy groups are perhaps the most important environment for our understanding of galaxy evolution, AGN and star formation driven feedback, and the development of the hot intergalactic medium (IGM). Past studies of groups have tended to approach these issues either by using optical selection and studying the galaxy populations, or using X-ray selection to find suitable targets for IGM studies. Both approaches suffer from biases, and we therefore created the Complete Local-Volume Groups Survey (CLoGS), an optically-selected statistically-complete sample of 53 groups in the nearby Universe (D<80 Mpc), for which we have carried out a programme of observations in both low-frequency radio (using GMRT at 610 and 235 MHz) and X-ray wavebands (using XMM-Newton and Chandra). We have completed observations of the 26-group high-richness subsample, itself statistically complete, and here present the initial results of the X-ray survey. We find a wide variety of group properties, in terms of galaxy population, hot gas content, and AGN power. We will discuss the relative fraction of cool-core and non-cool-core groups in the sample, the range of AGN activity observed in the dominant galaxies, and the degree of disturbance observed in the IGM, and compare these properties with the results of other surveys.

PARKER LauraOral
The connection between galaxies and their halo environment
We investigate the dependence of star formation rate and morphology on host halo properties for a large sample of SDSS group and cluster galaxies. We find that galaxy populations in groups with strong X-ray emission have suppressed star formation and fewer disc-dominated galaxies, at fixed galaxy stellar mass and host halo mass. We further explore the dependence of galaxy properties on group dynamics for galaxies within the virialized regions of groups and clusters and compare those to infalling populations. We show that the fraction of both star-forming and disc galaxies are independent of host dynamical state for infalling galaxies, while galaxies within the virial radius are sensitive to the dynamical state of their host. Together these findings help constrain the mechanisms at play in environmentally driven galaxy evolution.

PONMAN TrevorOral
Meeting summary

The halo mass, a key ingredient to understand the galaxy evolution.
The most striking feature of the Cosmic Star Formation History (CSFH) of the Universe is a dramatic drop of the star formation (SF) activity, since z~1. In this work we investigate if the very same process of assembly and growth of structures is one of the major drivers of the observed decline. We study the contribution to the CSFH of galaxies in halos of different masses. This is done by studying the total SFR - halo mass - redshift plane from redshift 0 to redshift z~1.6 in a sample of 57 groups and clusters by using the deepest available mid- and far-infrared surveys conducted with Spitzer MIPS and Herschel PACS and SPIRE. Our results show that low mass groups provide a 60-80% contribution to the CSFH at z~1. Such contribution declines quickly in the last 8 billion years to less than 10% at z~0. The large contribution at z~1 is due mainly to very massive, highly star forming Main Sequence galaxies favoring the group environment. Below z~1 a quenching process must take place in massive halos to cause the observed faster suppression of their SF activity. Such process must be a slow one, as most of the models implementing a rapid quenching of the SF activity in accreting satellites significantly underpredicts the observed SF level in massive halos at any redshift. Starvation or the transition from cold to hot accretion would provide a quenching timescale of 1 Gyrs more consistent with the observations.

PRASAD DeovratPoster
AGN Feedback in Galaxy Groups
Various studies have shown that stellar feedback is insufficient to overcome the cooling flow in galaxy groups. AGN feedback can balance the cooling flow in such systems. Our cluster studies show that condensation of cold gas, and the consequent enhanced accretion, is required for AGN feedback to balance radiative cooling, and to match the observed cool core properties. Using high resolution 3D hydrodynamic simulations, we study the evolution of galaxy group cores heated by feedback-driven bipolar active galactic nuclei (AGN) jets. We study the gas kinematics (cold and X-ray) and see the influence of AGN feedback on how far the hot gas is pushed out relative to the dark matter halo. In particular, we find that substantial amount of hot gas can be pushed beyond the virial radius in galaxy groups. We calculate the temporal evolution of gas to total mass ratio at different radii and times. We also study the relation between jet power and X-ray luminosity, and compare with our cluster simulations.

RABOLD ManuelPoster
Precision Cosmology through numerical simulations of galaxy groups
I will report on the results from the latest state of the art cosmological simulations of galaxy group size halos. The main focus of our analysis lies on accounting for the effects baryonic physics onto the mass distribution and hence the power spectrum, in prospect of the precision required for upcoming surveys like EUCLID. Our simulation suit consist of 16 galaxy group size halos, which we simulated with various underlying models: from pure n-body and adiabatic hydro simulations, to simulations including different stellar feedback properties and the latest implementation of AGN feedback in the RAMSES code.

REMUS Rhea-SilviaPoster
Galaxy Group Assembly: The Different Paths to Group Formation and their Connection to the Type of Group
Observationally, groups can be classified as loose, compact or fossil. From a simulation point of view, groups are usually selected by identifying halos with total masses between $10^13-10^14M_sun$ at z=0. We use a combination of groups selected from a high-resolution dark-matter-only cosmological simulation together with groups extracted from a set of very large hydrodynamical cosmological simulations called Magneticum to study the impact of the formation history as well as the environment on shaping the different types of observed galaxy groups. This combined approach allows to explicitly study which group properties are driven by the dark matter component alone and which properties are determined by the baryonic component within the groups. I will present the results of this study, especially focusing on the fossil and compact groups in the sample, and discuss their different formation channels and the implications for the massive central galaxies as well as the galaxy populations in groups. I will also focus on the role of the extended stellar halos in these different types of groups and how the groups bridge the regime of intra-cluster light found in massive clusters and extended stellar halos of galaxies.

ROSANI GiulioPoster
The Influence of Galaxy Environment on the Initial Mass Function of Early-Type Galaxies
Recent findings have shown that massive ellipticals have an excess in low-mass stars with respect to the average Milky Way distribution. For this reason their stellar initial mass function is referred to as bottom-heavy. Massive ellipticals, and more generally early-type galaxies, are preferentially found in denser environments, which affect their formation and evolution, for example by favouring their size growth since z~1. This raises the question of whether environment has also some influence in shaping the IMF of early-type galaxies. We have tested this by measuring IMF-sensitive line indices from SDSS spectra of a sample of ~21000 bona-fide early-type galaxies at z<0.1, for which the information on environment and hierarchy is available through the group catalogue of Yang et al. (2007). In this talk we will present the results of such analysis constraining the impact of environment on the IMF of early-type galaxies.

SMITH RoryPoster
The consequences of group preprocessing for galaxies in clusters: Insights from cosmological simulations
Using a set of 8 high resolution cosmological simulations of clusters of galaxies, we identify a sample of galaxy groups at high redshift, with similar mass to the Local Group. We find that the group members suffer a wide range of mass loss, from very strong to very weak, depending on their orbital behaviour. By redshift zero, many of the groups fall into the galaxy cluster, and some lose group members to the cluster tides. Perhaps surprisingly, we find the tidal mass loss of the group does not dictate whether a group member will be unbound. Instead the distance of the group member from its group, as it passes the cluster centre, is a strong determining factor. As a result, we find galaxy groups tend to maintain their least affected group members, while releasing their most tidally damaged members into the cluster. Through this process, groups can effectively pollute the cluster galaxy population with heavily pre-processed galaxies.

SMITH GrahamOral
The masses of groups of galaxies

Hierarchical Structure Formation at Low Masses: The Discovery of Dwarf Galaxy Groups
A key predction of the Lambda Cold Dark Matter paradigm is that the merging of smaller galaxies to form larger ones should also occur at lower mass scales, but direct observational evidence has remained elusive. We present an example of hierarchical structure formation at low mass scales caught in action: the discovery of the first compact groups of galaxies that contain only low mass, dwarf galaxy members. As part of an effort to understand the dwarf-dwarf merger sequence, we are conducting TiNy Titans (TNT), a panchromatic survey of a sample of nearby, interacting dwarf galaxy pairs. In searching the Sloan Digital Sky Survey for isolated dwarf-dwarf galaxy interactions (> 1.5 Mpc from a massive galaxy), our systematic search has revealed not just dwarf pairs but also seven groups of galaxies that only contain low mass (5 x 10^6 Msun < Mstar < 10^9 Msun) dwarf members. To confirm their status as bound groups, we have obtained followup observations in the form of very narrowband (10 Angstrom) Halpha imaging with the Maryland Magellan Tunable Filter Fabrey Perot on Magellan, longslit optical spectroscopy with the Apache Point Observatory, and moderate resolution neutral hydrogen gas maps. Using the total stellar and HI masses of the known group members, and assuming a dark matter mass-to-light ratio of 10, the group member velocities are far lower than the predicted escape velocity, thus indicating a bound system. The seven groups found by TNT build upon the discovery of looser associations of Tully et al. 2002 and Tully et al. 2006, associations with much larger physical separations, velocity separations, and mass-to-light ratios.

TOLLET EdouardOral
How much of their stellar mass do group/cluster galaxies lose to the ICL ?
This work presents a semi-phenomenological analysis of the stellar mass fraction that galaxies lose to the intracluster light (ICL) through tidal stripping since entering the group/cluster environment. The stellar mass stripped from galaxies is at least as large as the difference between the stellar mass at the time of entry and that at z = 0. We estimate the former from stellar mass – halo mass relation at the redshift of entry and the latter by matching the conditional stellar mass function of satellite galaxies and the conditional mass function of dark matter subhaloes for a same group mass. To follow the positions and merging histories of galaxies within groups and clusters from the redshift of entry to z = 0, we track haloes and subhaloes in a cosmological N-body simulation. When a subhalo falls under the resolution of the halo finder, its trajectory is computed by integrating equations of motion that include dynamical friction. We are thus able to distinguish between continued mass accretion, tidal stripping, and starvation and find that the respective roles these processes depends critically on group/cluster mass. The results are compared with observations of the mass in the ICL as a function of host halo mass.

Quenching the star formation in galaxies up to large clustercentric distances
It is well known that galaxy properties such as their specific star formation rate depend on their environment, with quiescent galaxies residing preferentially in high-density regions. However, it is not clear how far from the cluster center we still see quenching, and what is the role of pre-processing in small groups, in filaments, or simply small groups within filaments. To shed light on these questions, we use an SDSS-based sample of groups and clusters defined with a state-of-the-art group finder algorithm to determine how the fraction of quenched galaxies, f_q, varies with the distance from the cluster center up to 10 r_vir. We find that f_q decreases smoothly up to much further distances than what was previously thought. We quantify how the radial variation of f_q also depends on galaxy line-of-sight velocity, as well as on the galaxy and group masses. By comparing different approaches to assigning galaxies to the groups and the radial variation of f_q along different directions, we investigate the role of pre-processing in small groups and in filaments.

TULLY BrentOral
Your Average Group
Most groups are small and the place to look for small groups is very nearby. Accurate distances to almost 400 galaxies within 10 Mpc from tip of the red giant branch measurements are providing good discrimination of nearby groups.

VAN DER BURG RemcoPoster
The abundance and spatial distribution of ultra-diffuse galaxies in nearby galaxy clusters
Recent observations have shown that Ultra-Diffuse Galaxies (UDGs, which have the luminosities of dwarfs but sizes of giant galaxies) are surprisingly abundant in local clusters of galaxies. The origin of UDGs remains unclear, since one would naively expect them to be easily disturbed by tidal interactions in the cluster potential. The interpretation of these studies has been hindered by the (partly) subjective selection of these galaxies, and the limited study of only the Coma and Virgo clusters. I’ll present results of the first systematic search for UDGs in eight nearby clusters. I’ll focus on the abundance of UDGs as a function of halo mass, and their radial distribution in the clusters. The latter indicates that they were likely accreted by the cluster several Gyrs ago, but are likely still hosted by massive dark-matter haloes. I’ll discuss implications and future prospects to learn more about the properties and formation histories of these mysterious galaxies.

Unveiling the Hydrodynamics of Galaxy Transformation in Groups: The Survival and Destruction of Interstellar Medium Gas
Galaxies in harsh group and cluster environments undergo a variety of transformation processes which strip them of gas and suppress star formation, turning them into `red and dead' galaxies. Group environments significantly contribute to cluster galaxy evolution, since they can pre-process up to 40% of eventual cluster galaxies, and during group-cluster mergers remain bound with their galaxies subject to enhanced stripping and harassment. Galaxy transformation in dense group environments is far from straightforward; a large fraction of these galaxies have hot X-ray emitting coronae that have survived stripping and thermal conduction, exhibit star formation in their stripped coherent tails and disks, and have active galactic nuclei. Understanding the balance between process that remove gas (tidal and ram pressure stripping, thermal conduction) and the processes that help retain and replenish gas in galaxies (magnetic fields in the intragroup medium, radiative cooling, and feedback) requires specialized hydrodynamic simulations. While there is abundant observational evidence that these processes operate in group and cluster environments, a theoretical understanding of the physics controlling the energy cycle in their galaxies remains elusive. I will present results from some of these simulations that include, to date, the effect of ram pressure stripping on removing group galaxies' hot gas, the effect of magnetic fields on this process, and the effectiveness of isotropic and anisotropic thermal conduction in removing and alternatively retaining galactic gas. Additionally, group galaxies themselves affect their host environments, by amplifying their magnetic fields, injecting turbulence, and injecting metals into the intragroup medium; I will quantify the importance of these phenomena.

VOIT MarkOral
AGN Feedback and Regulation of the Intragroup Medium
X-ray observations of galaxy groups show that their entropy profiles deviate from what gravitational structure formation alone would produce. These deviations result from radiative cooling and the feedback processes that oppose cooling. I will discuss the role of AGN feedback in regulating intragroup cooling, focusing in particular on models in which feedback maintains the hot ambient gas in a state that is marginally unstable to condensation. Such models are proving quite promising in explaining both the X-ray scaling relations of groups and their radial entropy profiles.

WANG TaoPoster
Insights into the formation of group/cluster galaxies from the most distant X-ray cluster at z=2.506
We have recently discovered a remarkable massive galaxy concentration at z=2.506, which is confirmed to be the most distant (X-ray) cluster known to date. Furthermore, this cluster exhibits both an X-ray emitting halo and a huge concentration of starbursts (as revealed by ALMA, JVLA and IRAM-NOEMA observations). These unique properties suggest that it was caught shortly after formation, providing a rare chance to directly constrain the physics of galaxy (trans)formation in the densest environment. In this talk, I will present what we have learned on the star formation and molecular gas properties of the member galaxies of this cluster, and the implication on massive galaxy/cluster formation in the early epoch.

WOO JoannaOral
Environment Quenching and Morphological Transformation
It is well established that environment plays a role in the quenching of star formation in galaxies. In addition, it is becoming increasingly clear that the stars in quenched galaxies in dense environments have prominent bulges, or equivalently, are centrally concentrated/compact, having high central stellar mass surface densities. I will present new work that explores the connection between these two effects, namely environment quenching and the build-up of galaxy inner densities. Using a diagnostic diagram of specific star formation rate (sSFR) vs the stellar surface density within 1 kpc (Sigma_1kpc) in different environments, we have found that satellites are distributed differently in this space than galaxies in the field, giving clues as to the satellite quenching track. In particular, green valley satellite galaxies are less compact than green valley field galaxies of the same mass. Furthermore, the star-forming ``main sequence'' of satellites is lower in sSFR compared with the field. In addition to these differences between satellites and field, one feature of the sSFR-Sigma_1kpc diagram is shared between all environments: quenched galaxies are always more compact than star-forming galaxies such that there are no quenched galaxies with low Sigma_1kpc. These findings can be interpreted as a form of compression that increases Sigma_1kpc occurring only after environmental quenching. Alternative scenarios in which Sigma_1kpc is untouched include tidal stripping of outskirts from a higher mass bin and satellite "progenitor bias" in which quenched satellites with high Sigma_1kpc were quenched early. I will present new MUSE observations of a group of galaxies that attempts to break this degeneracy, showing direct evidence for the former scenario, i.e., for environmentally triggered compression.

X3JHZ8 CLUZ3gPoster

ZOU SiweiPoster
The X-ray luminosity temperature relation of a complete sample of low mass galaxy clusters
We present Chandra observations of 23 galaxy groups and low-mass galaxy clusters at redshift between 0.03 and 0.15 with a median temperature of ~ 2 keV. The sample is a statistically complete flux-limited subset of the 400 deg^2 survey. We investigated the scaling relation between X-ray luminosity (L) and temperature (T), taking selection biases fully into account. In combination with other recent studies of the LT relation we show that there is no evidence for the slope, normalisation, or scatter of the LT relation of galaxy groups being different than that of massive clusters. Thanks to our rigorous treatment of selection biases, these measurements provide a robust reference against which to compare predictions of models of the impact of feedback on the X-ray properties of galaxy groups.

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