Galaxy groups

Michael BaloghDepartment of Physics and Astronomy

University of Waterloo

Outline

1. Where do groups fit in the hierarchy?

2. Group selection methods3. Properties of galaxies in groups4. Theoretical challenges

What is a group?

• ~few L* galaxies

• Mhalo~1012-5x1013 (<500 km/s) At higher masses, galaxy population

seems to be weakly dependent on halo mass

• Physically associated – but not necessarily virialized

Buildup of structure

• Most galaxies today are in groups

• Abundance evolves strongly

• Fraction of galaxies in groups (N>6) increases by about a factor 3 since z=1

Knobel et al. (2009)

Satellites/Centrals

• Nice idea• Which one is the

central galaxy?• Not surprisingly:

data show little difference between correlations of satellite/central galaxies

Satellites/Centrals

• Nice idea• Which one is the

central galaxy?

Satellites/Centrals

• Nice idea• Which one is the

central galaxy?• Not surprisingly:

data show little difference between correlations of satellite/central galaxies

Halo massWeinmann et al. (2006)

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=1014.2 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

Fra

ctio

n of

pro

cess

ed g

alax

ies

Halo massMcGee et al. (2009)

Slow truncation

• And z evolution would be rapid

Fra

ctio

n of

pro

cess

ed g

alax

ies

Halo massMcGee 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 (also: Ellingson et al. 2001)

Halo mass McGee et al. (2009)

Group Selection Methods

• Redshift surveys• X-ray• Photometric surveys

Redshift surveys

• 2dFGRS/SDSS/6dF >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)

• “For satellite galaxies, a nearly equally strong dependence on halo mass and stellar mass is seen.”

Kimm et al. 2009

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• At fixed stellar mass, groups have fewer

blue galaxies than the field

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• 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)

Halo mass is King(?)

• Galaxy properties at z=0 depend almost equally on stellar mass and halo mass

• SFR/SN/AGN/gas accretion rate are the king’s advisors Theorists have worked hard

to make models that work, assuming they correlate primarily with halo mass.

Does success in matching the luminosity function under this assumption mean we know what’s going on?

• Or have we just given the emperor a new set of clothes?

Conclusions/Future Directions

• 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?