Galaxy groups

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

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

3. What is a group? • ~few L* galaxies • Mhalo~1012-5x1013 (s<500 km/s) • Physically associated – but not necessarily virialized • At higher masses, galaxy population seems to be weakly dependent on halo mass

4. 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)

5. 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)

6. Cluster growth via groups • M=1014.2 clustersaccrete 35% of galaxies via groups • For Coma-like clusters, fraction is 50%. McGee et al. (2009), using Font et al. (2008) model

7. 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.

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

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

10. 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 McGee et al. (2009) Halo mass

11. Group Selection Methods • Redshift surveys • Xray • Photometric surveys

12. 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

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

14. 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)

15. 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)

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

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

18. Group properties

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

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

21. Properties of X-ray groups • Spiral fraction in X-ray groups correlates with s, 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)

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

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

24. 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)

25. 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)

26. Theoretical challenges

27. 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)

28. 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)

29. 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?