Report on the Conference on Groups of Galaxies in the Nearby Universe held in Santiago de Chile, 5–9 December 2006

Ivo Saviane, Valentin D. Ivanov, Jura Borissova (ESO) n Bi

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For every galaxy in the field or in clusters, pe p

there are about three galaxies in groups. ou

Therefore, the evolution of most galax- Gr r

ies actually happens in groups. The Milky pe

Way resides in a group, and groups can be found at high redshift. The current xies 1 generation of 10-m-class telescopes and Gala

space facilities allows us to study mem- of bers of nearby groups with exquisite de- tail, and their properties can be corre- –1 Number L < 41.7 Log (erg s ) lated with the global properties of their x L > 41.7 Log (erg s–1) host group. Finally, groups are relevant x for cosmology, since they trace large- scale structures better than clusters, and –22 –20 –18 –16 –14 Absolute Magnitude (M ) the evolution of groups and clusters may B be related. Figure 1: Cumulative B-band luminosity function of Strangely, there are three times fewer pa- 25 GEMS groups of galaxies grouped into X-ray- bright and X-ray-faint categories, fitted with one or pers on groups of galaxies than on clus- two Schechter functions, respectively (Miles et ters of galaxies, as revealed by an ADS al. 2004, MNRAS 355, 785; presented by Raychaud- search. Organising this conference was a hury). Mergers could explain the bimodality of the way to focus the attention of the com- luminosity function of X-ray-faint groups. munity on the galaxy groups. We also wanted to offer a venue where people All you wanted to know about groups The evolution of low-velocity dispersion coming from various research fields could (but were afraid to ask) groups is dominated by mergers, which meet and discuss groups from differ- could explain the bimodal mass func- ent perspectives. All this happened in a Groups are bound structures with tion of the X-ray-faint groups (X-ray-faint 12–14 friendly atmosphere created by Hotel Tor- masses in the range M = 10 MA (Eke), groups tend to have low-velocity disper­ remayor in Santiago. containing less than fifty galaxies (Conse- sion, and vice versa), if intermediate- lice), and with typical sizes of a few Mpc. mass members merge to build the largest The discussion was organised in seven Groups detected in X-rays have luminos- group members (Raychaudhury). The bi- 41–43 –1 sessions, introduced by invited reviews: ities of LX = 10 erg sec and gas modal mass function (see Figure 1), simi- Eva Grebel (Local Group versus Near- temperatures of kT = 0.1–3 keV (Ponman). lar to that of clusters, is confirmed in by Groups), Vince Eke (Groups Searches Most of the stellar mass in the present compact groups (Bomans). Compared to and Surveys), Chris Conselice (The Evo- Universe is in groups similar to the Local compact groups, the loose ones tend to 12 lution of Galaxies in Groups – Obser­ Group with masses ~ 2 × 10 MA and have fewer low-mass members. 14 vations), Gary Mamon (The Evolution of only 2 % is in clusters with M > 5 × 10 MA Galaxies in Groups – Theory), Ann Zab- (Eke). Groups were already present at The results presented here were obtained ludoff (Evolution of Groups as Systems), redshifts z > 1 (Conselice). Cosmological thanks to large observational efforts Trevor Ponman (Interstellar Medium and simulations predict a much larger num- (Table 1). Historically, the first group cata- Intragroup Medium), Stefano Borgani ber of galaxy satellites than observed, and logues were biased toward compact (Groups in a Cosmological context), and H i high-velocity clouds cannot fill in this groups, which are the easiest to identify finally Ken Freeman (Conference Sum- gap (Pisano). Groups follow a fundamen- from imaging surveys. Modern redshift mary). There were almost 50 contributed tal plane (Muriel), and the most massive surveys allow selections including reces- talks and 30 posters. Most speakers ones have an X-ray halo with an extended sion velocities, and finding algorithms agreed to share their presentations with component (Zabludoff). can be tested on mock catalogues gen- the astronomical community at http:// erated with dark matter (DM) simulations www.sc.eso.org/santiago/science/NGG/ A special class of groups are the so- (Eke). finalprogram.html. Here we give a short called ‘fossil groups’ – isolated ellipticals summary of the main conference ideas, with properties similar to a group, which mostly based on the invited reviews. could be the final stage of a collapsed Galaxies in groups group. However, most isolated ellipticals are not collapsed groups (Forbes). There Galaxies in groups can be affected by are only 15 fossil groups known to date. processes like ram pressure stripping, in-

The Messenger 123 – March 2006 63 Other Astronomical News Saviane I. et al., Conference on Groups of Galaxies in the Nearby Universe

Table 1: Summary of the state-of-the-art group catalogues and surveys discussed at the meeting.

Name Description Reference 160 and 400 square-degree ROSAT 14 groups were studied to compute the mass and compare mass-to-light ratios Vikhlinin et al. 1998, ApJ 502, 558 surveys with simulations (presented by A. Hornstrup) 2PIGG (2dFGRS Percolation-Inferred The largest available homogeneous sample of galaxy groups, public Eke et al. 2004, MNRAS 348, 866 Galaxy Group) catalogue ALFALFA (Arecibo Legacy Fast Alfa Large-scale survey of extragalactic H i over 7000 square degrees of sky, up to Giovanelli 2005, AAS 207, #192.03 (= Arecibo L-Band Feed Array)) survey cz = 18 000 km/s. Spectral resolution is 5 km/s. It can detect H i clouds with more 7 than 10 MA throughout most of the Local Supercluster AMIGA project (Analysis of the Inter- Multiwavelength database of isolated galaxies, including optical (B and Ha), Verdes-Montenegro et al. 2005, A&A stellar Medium of Isolated Galaxies) infrared (FIR and NIR) and radio (continuum plus H i and CO lines) 436, 443 CNOC2 (Canadian Network for Obser- Spectroscopically selected catalogue of 200 groups at intermediate redshift over Carlberg et al. 2001, ApJ 552, 427 vational Cosmology) survey 1.5 square degrees on the sky GEMS (Group Evolution Multiwave- Catalogue of 60 galaxy groups at 15–130 Mpc distance. Plus: X-ray (ROSAT presented by Forbes length Study) project PSPC 10 000 sec, 1.5 degrees), optical imaging (0.5 degrees), Parkes H i mapping (5.5 degrees), ATCA H i follow-up, 6 dFGS spectra, 2 MASS K-band photometry, XMM/Chandra imaging, Mock catalogues H i survey of six loose groups Observations of six spiral-rich, loose groups between 10.6–13.4 Mpc, presented by Pisano over 25–35 square degrees: Parkes Multibeam and ATCA; Mass sensitivity 5 of 5–8 x 10 MA LVHIS (Local Volume H i Survey) H i imaging of all nearby (distance less that 10 Mpc), gas-rich galaxies; presented by Koribalski deep 20-cm radio continuum imaging with ATCA and VLA; deep H-band and Ha imaging Sharc (Serendipitous High-redshift 638 ROSAT PSPC observations with |b| > 20 degrees and exposure time greater Romer et al. 2000, ApJS 126, 209 Archival ROSAT Cluster) survey than 10 000 seconds; total 178.6 square degrees; found the most distant fossil (presented by F. Durret) group at z = 0.59 teractions and harassment, mergers, at lower redshifts. Mergers occur mostly gas become part of the intragroup group tidal field, gas loss and suppressed at redshifts z > 1: for example at z = 2.5 ­environment. Eventually, common dark star formation (also known as strangu- about 50 % of bright galaxies are un- matter and hot (X-ray) gas halos are lation or suffocation). Merging is the most dergoing mergers, while today only 2 % formed (Zabludoff). The X-ray emission important of them because of the low of galaxies merge per Gyr (Conselice). increases, and the X-ray halo becomes relative velocities of galaxies in groups in Most of the stars in group members also more and more regular. Later, the dif- comparison with the galaxies in clusters. formed between redshifts z = 2.5 and 1. fuse DM distribution will reduce the mer­ Simulations show that mergers induce an ger rate and moderate the evolution of intense and brief (of the order of a hun- Locally, environmental effects can be groups. At least a fraction of groups end dred Myrs) surge of star formation before traced directly by reconstructing star-for- their lives as fossil groups. the final coalescence into a spheroid, mation histories of individual galaxies. which evolves passively afterwards. Simul- For example, the fraction of intermediate- Most low-redshift groups are just detach- taneously, mergers transfer momentum age stars of Milky Way dwarf satellites ing from the Hubble flow, as suggested from the interacting galaxies to the group depends on their distance from the by the time evolution of the virial mass-to- as a whole, thereby increasing the group Galaxy. On the contrary, this fraction is light ratio (Mamon). In particular, the de­ velocity dispersion. Indeed, observa- constant in M81 satellites (Da Costa), tachment for the Local Group occurred at tions show that there are more spheroids probably due to the compactness of the z < 0.7 (Freeman). The mass-tempera- in groups with higher velocity dispersion M81 group, where multiple close en- ture and mass-luminosity distributions in (Zabludoff). counters have homogenised their star- the X-rays for clusters and groups can formation histories. constrain the cosmological parameters Eventually, the feedback from the resid- (Borgani). ual black-hole and active galactic nucleus (AGN) reduces the star formation by a The evolution of groups To summarise, as a group evolves, the factor of ten or more. At least 50 % of gal- dwarf-to-giant ratio, early-type galaxy axies in compact groups are low-lumi- The origin of groups is probably related fraction, intragroup starlight and metal- nosity AGNs (Martinez), while the field to large-scale gaseous filaments at licity, the velocity dispersion, and the fraction is only 30 %. Moreover, the cores high redshift. Before virialisation, smooth mass of the central giant elliptical grow. of X-ray groups are often disturbed, which accretion, supernovae and AGN acti- The metallicity of the intragroup medium could be additional evidence for AGN vity enhance the entropy, and the metal- also increases thanks to the intragroup feedback (O’Sullivan). The selective sup- enriched gas cannot be retained by the stars, whose ejecta do not have to over- pression of star formation in larger group shallow potential of pre-collapse groups come galactic potential wells (Zabludoff). members could explain the downsizing (Ponman, Borgani). During the viriali- phenomenon – the decrease of the maxi- sation, the central spheroidals grow via Observations are consistent with this sce­ mum luminosity of star-forming galaxies mergers. Early-type stars and enriched nario. As mentioned above, groups with

64 The Messenger 123 – March 2006 higher velocity dispersions have higher fractions of early-type galaxies. And the intragroup medium can be responsible 0.6 for stripping, e.g. of NGC 2276 in the NGC 2300 group (Ponman). In turn, strip- ping enhances the fraction of passive galaxies in groups. Further observational 0.4 support for this evolutionary scheme action Fr are the constant radial profiles of velocity piral dispersion, which point to a common S DM halo. Next, if the early enrichment his- 0.2 tory of the intragroup gas is dominated by type II supernovae, and the late histo- ry by type Ia supernovae, then this could explain the observed decrease of the 0 overall metallicity toward the outskirts of 0 1 2 the group, and the alpha-enhancement Log Surface Density (Mpc–2) in the outer parts of groups (Rasmussen) ­ Figure 2: Spiral fraction (including irregulars) as a merging rates, respectively (for clarity the error bars because the early ejecta had time to function of surface density (averaged over radial bins). are omitted from these points). Direct type mergers spread across the group. The histogram represents the local clusters, and the in groups convert late-type galaxies into spheroidal open circles with error bars are the group data. galaxies more efficiently than grazing mergers in Crossed and shaded circles represent group points clusters (Helsdon and Ponman 2003, MNRAS 339, after estimated corrections for 3D density and L29; presented by Mamon). Groups and clusters of galaxies ter light in Virgo probably originates in These few paragraphs can only give a It was realised during the conference that tidal interactions inside group-size struc- brief sense of the stimulating discussion groups are important for the evolution tures (Mihos), favoured by their low veloc- during the five days of the conference, of clusters as well. Clusters may grow by ity dispersion. Since tidal features are and we hope that all participants went accretion of groups, as exemplified by erased as clusters evolve, the presence away with fresh views on the current sta- the Eridanus Super-group infalling toward of such features would indicate that tus of galaxy groups studies. The pro- Fornax (Brough). Therefore, some cluster the cluster is dynamically young, still ‘frag- ceedings will be published later this year properties might be explained by groups, mented’ in groups. in the ESO Astrophysics Symposia series. such as the X-ray medium, high dwarf- to-giant galaxy ratio, brightest cluster gal- Although mergers can happen both in axies, and the early-type galaxy frac- clusters and groups, the high veloci- Acknowledgements tion (especially in more massive groups). ty dispersion in clusters leads to less ef- We would like to express our gratitude to the mem- ficient orbital-decay-type mergers, while bers of the SOC, and especially to the SOC chair Likewise, the evolution of galaxies in clus- more efficient, direct head-on mergers Duncan Forbes, for their efforts that made it possible ters might be dominated by group-scale are common in groups, especially the to organise this meeting. Last but not least, the environment, driving e.g. the morphology- evolved, X-ray bright ones (Mamon). This ­success of the conference would have been impos- sible without the excellent work of Maria Eugenia environment relation, the Butcher-Oemler can explain the higher fraction of early- Gómez, Paulina Jiron and Ismael Martínez, and the effect, and the brightest cluster galax- type galaxies in this class of groups com- financial support of ESO. ies formation. For example, the intraclus- pared to the field and clusters (Figure 2).

Almost all conference participants can be seen in this photograph, taken in the hotel frontyard. The excep- tion is Valentin Ivanov, who’s taking the picture!

The Messenger 123 – March 2006 65