The important role of galaxy groups

The important role of galaxy groups

Michael BaloghDepartment of Physics and Astronomy

University of Waterloo

Galaxy Clusters• A standard picture to motivate environmental

effects: Clusters are dominated by bright, red ellipticals

Massive galaxies• Not really

environmental effect: just result of biased galaxy formation Brightest galaxies

form early, rapidly SCUBA galaxies are

likely proto-ellipticals in clusters

Bower et al. 2006

Quenching

• Virial shocks become important above ~1012 Msun and after z~2

• But still require some mechanism to shut off star formation Must operate at ~1012

Msun to get CMD right AGN? Clumpy

accretion? Other things?

Dekel & Birnboim 2006

Low-mass galaxies• Galaxies with M~109 MSun are well below the

“threshold” mass.• But the fraction of blue galaxies depends

strongly on environment.

Baldry et al. (2006)Haines et al. 2007

Possible environmental effects

• Primordial? Dark matter mass accretion rate depends on environment

(Maulbetsch et al. 2006). Squelching of galaxies that form after reionization (Tully et al.

2002)• Enhanced merging, harassment• Ram-pressure (or tidal) stripping of cold gas• Strangulation: ram-pressure stripping, or shock heating, of

galaxy corona

• Most models currently include primordial plus merging effects, and a simplified strangulation prescription.

Direct evidence of environmental effects

Kenney et al. 2003Vollmer et al. 2004

• Ram-pressure stripping in Virgo

H for Virgo galaxyH for normal galaxy

• Truncated H disks in cluster spirals

• These galaxies have -20<MB<-18

Koopmann & Kenney 2004also: Vogt et al. 2004

• Passive Spirals• S0, dSph, UCDs• Wolf’s dusty spirals?

Peak in infall region?

Environmental quenching

• Simple galaxy formation models assume that gas accretion shuts off immediately when haloes merge

• Naturally leads to “downsizing” as low mass galaxies are incorporated into more massive haloes This environmental effect is a crucial ingredient!

Dekel & Birnboim 2006

Predictions•Isolated galaxies: only most massive galaxies are red.•Satellite galaxies at z=0 will be red, independent of halo mass, above ~1012 MSun.

•High red dwarf/giant ratio in groups and clusters•Little evolution in cluster and group colours to z=1

Isolated Galaxies

Incr

easi

ng s

tell

ar m

ass

• No low-mass, red galaxies, as predicted

Balogh, Salton et al. in prep.

Red sequence

threshold

Predictions• SFH of central galaxy depends on halo mass • SFH of satellite galaxies independent of

halo mass for M>1012MSun

Croton et al. 2006 models; from Weinmann et al. 2006

• Note: satellites are red even if they have low mass

• Fraction of late-type satellite galaxies independent of halo mass for M>1014 MSun Models do a

reasonable job at matching the most massive clusters (e.g.Diaferio 2001; Springel et al. 2001; Okamoto & Nagashima 2003)

Weinmann et al. 2006

Fra

ctio

n of

Lat

e-ty

pe g

alax

ies

Groups and clusters

The faint red galaxy problem!

• Models predict too many faint, red satellites in groups • This has a strong effect on the overall population, which

is dominated by groups

Weinmann et al. 2006

Red Galaxy luminosity function

• Dwarfs: -18.2>Mv>-20• Giants Mv<-20

Weinmann et al. haloes (SDSS)Courtesy: Sean McGee

De Lucia et al. (2007)

Red

Red Dwarf/Giant Ratio

• Faint red galaxies build up with time.

• Note: these “dwarfs” are MV~-19

VVDS: Zucca et al. 2006

COMBO-17: Bell et al. 2004

Driver et al. 2006Marzke & Da Costa 1997

PISCES: Tanaka et al. 2006

Field surveys


• Local clusters have more red dwarfs than the field.

• Clusters evolve strongly: faint red population appears at z<1 ?

Lu et al. 2007

De Lucia et al. 2007

Stott et al. 2007

Tanaka et al. 2007





Clusters

Gilbank et al. 2007


• Models get local clusters right!

Bower et al. 2006 (clusters)

Lu et al. 2007


Stott et al. 2007

Tanaka et al. 2007





Gilbank et al. 2007


• Oops. Too many faint red galaxies in the field!

Lu et al. 2007


Stott et al. 2007

Tanaka et al. 2007





Bower et al. 2006 (field)

Gilbank et al. 2007


• Models predict little evolution

• Strangulation is too effective in small groups, which are dominant at z<1

Models

Clusters

Field

Two solutions to the faint red galaxy problem

1. Mass threshold? 1013-1014 MSun works well for clusters

(Balogh, Navarro & Morris 2000; Poggianti et al. 2006)

Would improve match to the field Harassment, ram pressure stripping

should be stronger in these systems.

2. Timescale? Maybe it’s still strangulation, operating in

smaller haloes, but more slowly?

The importance of galaxy groups

• Masses 1013-1014 MSun

If there is a “threshold mass”, this is probably it.

• Common today and evolve strongly with redshift

• Should be close to the action, in time, even if the threshold mass is 1012 MSun. In rich clusters it seems we are observing most

galaxies long after their SF has shut down.

CNOC2:Groups at z~0.4• ~200 groups

between z~0.1 and z~0.55

• Follow-up at Magellan• 26 groups targeted between

z =0.3 and z=0.55

• Observations of 20 groups for 1 orbit each in F775W filter with HST ACS camera

• 3 Orbit GALEX data• IRAC and MIPS data

“CNOC2” GroupsZ=0.5

Millennium Simulation

All haloes

McGee et al. 2007

Evolution in Groups

• SFH of galaxies in groups are similar to the field, and evolve with it

Wilman et al. 2005

Groups @ z=0.5

• Active fraction weakly depressed relative to field

• No evidence for dramatic effects.

• Models predict much lower fractions

Balogh et al. 2006

Bower et al. model groups

Groups - morphology

• Use Gim2D to measure the fraction of light in the bulge (B/T)

• Low-z data from the MGC (Driver et al.)

• Models do well here. Merger history

OK. SFH needs work.

McGee et al. 2007

Black: dataRed: models

Two solutions to the faint red galaxy problem

1. Mass threshold? 1013-1014 MSun works well for clusters

(Balogh, Navarro & Morris 2000; Poggianti et al. 2006)

Would improve match to the field Harassment, ram pressure stripping

should be stronger in these systems.

2. Timescale? Maybe it’s still strangulation, operating in

smaller haloes, but more slowly?

Slow strangulation

• How quickly do galaxies lose their gas?• Consider analytic and numerical

(GADGET-2) models of “hot” gas+DM haloes merging with groups or clusters, on cosmologically sensible orbits.

McCarthy et al. 2007

Hot stripping in a uniform medium

• Instantaneous stripping: a fixed fraction of gas will be removed

• In reality there is a delay of ~1 Gyr which we model linearly:


Dark matter

Gas

Analytic prediction

sc

tM

Hot stripping in clusters

• Onset of stripping is delayed• =2, =2/3 works well for a

variety of orbits, mass ratios.• Takes ~2 Gyr to remove half

the gas mass Still plenty of hot fuel left The amount of gas left depends

on orbit, mass ratio etc., but the time delay of at least 1-2 Gyr is fairly robust


Observational evidence

• Sun et al. (2007) detect hot coronae around galaxies in clusters Reduced luminosity compared with isolated galaxies, but

still significant.

Summary

• There are environmental influences on galaxy formation after z=1

• Probably dominant in massive groups, not clusters.

• Current modeling of environmental effects is wrong and this has consequences for predictions of the general field (which is dominated by groups) Simple strangulation models may still work

well, if the instantaneous assumption is dropped.

Extra slides

Formation of massive galaxies in clusters

• At z>1 brightest galaxies are blue, clustered. Formation of

giant ellipticals Can be

understood in terms of cooling time, feedback, and biased galaxy formation.

DEEP2 (Cooper et al. 2006;2007)

Rich Clusters and bright galaxies

• Models give a pretty good match to rich clusters predict too few active galaxies in clusters, brighter

than MR=-20.7 But not a huge effect

SDSS clusters: Finn et al. 2007 Bower et al. 2006 models

Threshold mass• Difficult to match observations if all SF

shuts off above ~1012 MSun

Poggianti et al. 2006

SDSS clusters

1012 MSun threshold

Threshold mass• Difficult to match observations if all SF

shuts off above ~1012 MSun

Poggianti et al. 2006

SDSS clusters

1014 MSun threshold

Groups: Model predictions

• Bower et al. (2006) models predict groups should be more like clusters

• Too large a difference compared with the field

Group m

ean

Field

M/L ratios

• Compare total luminosity within R200 to dynamical mass (M200).

• Triangles include correction for passive evolution and dark halo mass growth. zavg ≈ 0.4

z ≈ 0 (Ramella et al. 2004)

Balogh et al. 2006

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The important role of galaxy groups