Driving Downsizing with groups of galaxies

Michael BaloghDepartment of Physics and Astronomy

University of Waterloo

or: the faint red galaxy problem

CollaboratorsDavid Gilbank, Sean McGee, Robbie Henderson

(Waterloo)Dave Wilman, Daniel Pierini (MPE, Garching)

Richard Bower, Simon Morris (Durham)John Mulchaey, Gus Oemler (Carnegie)

Outline

I. Review: Galaxy formation models

II. Evolution of faint red galaxiesIII. Galaxy groups at z=0.4IV. Revisiting starvation

The halo model

• The growth of dark matter structure is now well understood

• Galaxy formation history is tightly coupled to dark matter halo mass

www.nbody.net

The halo modelThe halo model

Radiativecooling

Hot baryons

Dark matter

~106 K for galaxies, hence invisible

>95% of baryons are dark

The inefficiency of star formationThe inefficiency of star formationstars= 0.0014 ± 0.00013

stars/ baryon =0.03

(Balogh et al. 2001; Cole et al. 2002)

Galaxy Luminosity Galaxy Luminosity FunctionFunction

Benson et al. 2003

Nu

mb

er

den

sity

of g

ala

xie

s

Luminosity

Theory

Data

stars/ baryon =0.03

(Balogh et al. 2001; Cole et al. 2002)

Stellar mass• Blue galaxies are absent above ~3x1010 MSun

• Star formation today occurs in low-mass galaxies

Kauffmann et al. (2003)

Baldry et al. (2004) 11 10 9 8 log M*

Gas Accretion• Halo mass scale

constant with time, ~2x1011 MSun.

• Separates “hot” and “cold” accretion (e.g. White & Frenk 1991)

• AGN feedback helps eliminate bright blue galaxies (Springel et al. 2005; Croton et al. 2006; Bower et al. 2006)

Dekel & Birnboim 2006

Galaxy Clusters• A standard picture to motivate environmental

effects: Clusters are dominated by bright, red ellipticals

Low-mass galaxies

• Galaxies with M~109 MSun are well below the “threshold” mass.

• But the fraction of red galaxies STILL depends strongly on environment.

Baldry et al. (2006)

Strangulation/Starvation

• Gas around satellite galaxies may be shock-heated, tidally- or ram-pressure stripped

• Stripping the cold, dense gas in the disk requires high velocities and ICM densities

• The hot halo can perhaps be stripped more easily (Larson, Tinsley & Caldwell 1980)

Kawata & Mulchaey 2007

Kenney et al. 2003Vollmer et al. 2004

Environment: models

• Standard assumption is that satellite galaxies instantly lose their entire hot halo. SFR then declines on a typical timescale

(Balogh, Navarro & Morris 2000):

• Low stellar-mass, red galaxies are predicted to be in groups, above the critical mass limit

GyrM

Mt

Sun

3.0

10*

102.2

Satellite galaxies at z=0• Most faint, satellite galaxies are blue• Models too efficient at shutting off gas

supply? Too rapid? Too complete? Or should this mechanism only apply to

massive haloes?

Weinmann et al. 2006

Model predictions

Part II: Evolution of faint red

galaxies

Red Galaxy luminosity function

• Faint red galaxies have appeared recently in clusters

De Lucia et al. (2007)

• Dwarfs: -18.2>Mv>-20• Giants Mv<-20

• Faint red galaxies have built up in clusters since z~1

Cluster data from:Gilbank et al. (2007)Stott et al. (2007)Hansen et al. (2007)Barkhouse et al. (2007)Andreon (2007)Tanaka et al. (2005)De Lucia et al. (2004)

Observed galaxy clusters

Gilbank & Balogh (2008) Redshift

Red

Dw

arfs

/Gia

nts

• Faint red galaxies are less common in the field – but also increasing with time (more rapidly?)

Field data from:Bell et al. (2003, 2004)Driver et al. (2006)Scarlata et al. (2007)Brown et al. (2007)Zucca et al. (2006)Baldry et al. (2004)

Observed galaxy clusters

Observed field galaxies

Gilbank & Balogh (2008) Redshift

Red

Dw

arfs

/Gia

nts

Observed galaxy clusters

Observed field galaxies

Bower et al. (2006) model predictions

Gilbank & Balogh (2008) Redshift

Red

Dw

arfs

/Gia

nts

• Models predict a large fraction of faint, red galaxies at all redshifts, even in the field

• Due to the red satellite galaxies in small groups

Gilbank & Balogh (2008) Redshift

Red

Dw

arfs

/Gia

nts

• The evolution in the field can be explained if faint, red galaxies are produced only in groups with masses greater than 1012.5 MSun.

SunM5.1210

SunM1310

SunM1210

Red dwarf/giant ratio

• Models are far too efficient at quenching star formation in satellite (group) galaxies

• Galaxy groups at z=0.5 are critical for detailed study of transforming galaxies

Redshift

Red

Dw

arfs

/Gia

nts

Part III: Galaxy groups at z=0.4

Groups at z~0.4• ~200 groups between

z~0.1 and z~0.55, selected from the CNOC2 survey (Carlberg et al. 2001)

• 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• XMM, Chandra

“CNOC2” GroupsZ=0.5

Millennium Simulation

All haloes

McGee et al. 2007

Star formation in groups

• At all stellar masses, star-forming galaxies are found less frequently in groups

Bower et al. model groups

Balogh et al. 2006

Fra

ctio

n w

ith

[OII

] em

issi

on li

nes

Passive galaxies

• Spitzer IRAC colours are an excellent tracer of low-levels of activity

Wilman et al. 2007

SpiralsE/S0

[8m]-[3.6m] colour

rest [m]

Star formation in groups• Dusty and/or low-levels of star formation in massive

galaxies Break occurs at ~1011 MSun. Group galaxies still show less activity than field galaxies

of the same mass.

10.5 11 11.5log10 Mstellar/MSun

Infr

ared

Act

ive

frac

tion

Opt

ical

ly A

ctiv

e fr

actio

n

Wilman et al. 2007 Balogh et al. 2006

Group morphologies

Fra

ctio

n of

dis

k ga

laxi

es

Fra

ctio

n of

dis

k ga

laxi

es

McGee et al. 2007

• Only a small difference in galaxy morphology at z=0.4 This evolves strongly to z=0 Suggest morphological transformation may lag behind

star formation quenching

CNOC2MGC

Allen et al. 2006

Passive spirals

• Moran et al. (2007) analyse GALEX colours of passive spirals in two rich clusters at z=0.5

• “starved” spirals appear to be found in infalling groups

GALEX• Starvation model seems a good fit to

the passive spirals in CNOC2 groups

McGee et al. in prep

CNOC2 groupsRed: passive spirals

Black: normal spiralsGreen: passive spirals

Blue: normal spirals

Summary: z=0.4 groups

• There is evidence galaxies are being quenched in groups, but the effect is not dramatic

• We are embarking on a full multiwavelength analysis from FUV to MIR to constrain the star formation histories of group members

Part IV: Revisiting starvation models

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

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:

McCarthy et al. 2007

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

• Through starvation alone, low-mass satellite galaxies could potentially continue star formation for a significant fraction of a Hubble time.

McCarthy et al. 2007

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

Cosmic Time

• buildup of mass on the red-sequence occurs with the most massive galaxies first

• decrease in the “quenching” stellar mass with redshift

Cimatti et al. (2006)

Universal relation

• Red fraction appears to depend on a simple linear combination of stellar mass and density

• Reflects the fact that stellar mass and density are correlated

Baldry et al. (astro-ph/0607648 )

Evolution in Groups

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

Wilman et al. 2005

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