Galaxy groups Michael Balogh Department of Physics and Astronomy University of Waterloo

Outline 1.Where do groups fit in the hierarchy? 2.Group selection methods 3.Properties of galaxies in groups 4.Theoretical challenges

What is a group? ~few L* galaxies M halo ~ x10 13 (  <500 km/s) Physically associated – but not necessarily virialized At higher masses, galaxy population seems to be weakly dependent on halo mass

Buildup of structure Group abundance evolves strongly Fraction of galaxies in groups (N>6) increases by about a factor 3 since z=1 Knobel et al. (2009)

Cluster growth via groups Clusters grow via:  Major mergers between clusters  Accretion of groups  Accretion of isolated galaxies Low-mass clusters may accrete much of their mass directly from the field Berrier et al. (2008)

Cluster growth via groups M= clusters accrete 35% of galaxies via groups For Coma-like clusters, fraction is 50%. McGee et al. (2009), using Font et al. (2008) model

Pre-processing Importance of groups also depends on how long these galaxies reside in group environment. And main progenitor was itself a group at some point.  Use “processed galaxies” as tracer of accretion histories.  Assume galaxies “transform” T Gyr after first accretion into a halo >M.

Slow truncation Without preprocessing: not only would groups be field-like, but clusters would show much more scatter Fraction of processed galaxies Halo mass McGee et al. (2009)

Slow truncation And z evolution would be rapid Ellingson et al. (2001) used this argument to support long (T~3Gyr) timescales from CNOC clusters Fraction of processed galaxies Halo mass McGee et al. (2009)

Group preprocessing Slow timescale, low mass threshold predicts:  Tight red sequence at z=0  Weak dependence on halo mass  Moderate evolution: negligible red fraction by z=1.5 Halo mass McGee et al. (2009)

Group Selection Methods Redshift surveys Xray Photometric surveys

Redshift surveys 2dFGRS/SDSS  >4500 sq degrees  >5000 groups with z<0.1 CNOC2  1.5 sq degrees  200 groups 0.2<z<0.55  Extensive follow-up of ~30 groups zCOSMOS  1.7 sq degree  800 groups 0.1<z<1 DEEP II  1 sq degree  899 groups with 2 or more members  0.7<z<1.4

X-ray selection: low-z ROSAT able to detect nearby systems with  ~100 km/s or greater  Zabludoff & Mulchaey (1998)  Osmond & Ponman (2004)  Rasmussen et al. (2008) Mulchaey & Zabludoff (1998)

X-ray selection: higher z XMM-LSS (~10 ks)  Willis et al. (2005) Mulchaey et al. (2007); Jeltema et al. (2007, 2008)  Nine X-ray groups at 0.2<z<0.6, from ROSAT DCS These probe low-mass cluster regime, but not true groups Mulchaey et al. (2006)

X-ray selection: higher z CNOC2 fields : Chandra and XMM data – combined depth equivalent to 469 ksec ( Chandra ) c.f. ~160 ks in COSMOS z=0.4 See also Knobel et al. (2009) Finoguenov et al. (in prep)

Photometric selection McConnachie et al. (2008) use SDSS to detect 7400 compact groups, photometrically. Attempt to correct for contamination using simulations

Photometric selection RCS: not effective in the group regime Completeness trusted down to  ~300 km/s. Gilbank et al. (2007)

Group properties

SDSS groups Weak correlation with halo mass for clusters Evidence for larger blue fractions in groups Bamford et al. (2009)

Low-mass satellite galaxies show dependence on halo mass on group scales Kimm et al Groups and clusters Also Weinmann et al. 2006, Pasquali et al. 2009

Properties of X-ray groups Spiral fraction in X-ray groups correlates with , Tx  X-ray bright groups tend to be spiral-poor (e.g. Brough et al. 2006)  Significant scatter in early fraction (Mulchaey & Zabludoff 1998) HI deficiency independent of X-ray properties in compact groups (Rasmussen et al. 2008) Osmond & Ponman (2004)

Groups at z=0.5 At fixed stellar mass, groups have fewer blue galaxies than the field Balogh et al. (2009)

Groups at z=0.5 Balogh et al. (2009)

Groups and clusters at z=0.5 Galaxies show a halo-mass dependence:  Red fractions of groups intermediate between cluster and field environments Balogh et al. (2009)

Low-sfr galaxies Wolf/Gallazzi Skibba 2008 Bamford? Virgo: Cowl & Kenney Hughes HCGs Hughes et al. (2009)

Low-sfr galaxies Mounting evidence that there may be a transition population of dust-reddened, low-sfr galaxies found in intermediate environments  STAGES supercluster: Wolf et al. (2008); Gallazzi et al. (2008) SDSS: Skibba et al. (2008); Bamford et al. (2008) Virgo: Crowl & Kenney (2008); Hughes et al. (2009) HCGs: Johnson et al. (2007); Gallagher et al. (2008)

Theoretical challenges

Rapid strangulation Compare z=0.5 group galaxy colour distribution with models  Narrow range of NIR luminosity Simple models overpredict the red fraction (but actually do a pretty good job) The blue galaxies are near the group halo – but not actually subhaloes Balogh et al. (2009)

Slow strangulation Models which slow the rate of transformation  Destroys distinct bimodality Maybe only a fraction of group galaxies should be affected; orbit-dependent? Puzzle: strangulation should be slow for low- mass galaxies (e.g. Haines, Rasmussen)… why so quick in GALFORM? Balogh et al. (2009)

Conclusions Robust samples of groups at 0<z<1 now routinely available  All require good mock catalogues to account for contamination, selection effects Need more precise measures of SFH  Dust-obscured star formation  SF on long vs short timescales Need to find source of scatter in group properties  Lx-M residuals? Concentration? Dynamics? Associated large-scale structure?