Driving Downsizing with groups of galaxies Michael Balogh Department of Physics and Astronomy University of Waterloo

or: the faint red galaxy problem Collaborators David 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 galaxies III.Galaxy groups at z=0.4 IV.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

The halo model Radiative cooling Hot baryons Dark matter ~10 6 K for galaxies, hence invisible

 >95% of baryons are dark The inefficiency of star formation  stars = ±  stars /  baryon =0.03 (Balogh et al. 2001; Cole et al. 2002)

Galaxy Luminosity Function Benson et al Number density of galaxies Luminosity Theory Data  stars /  baryon =0.03 (Balogh et al. 2001; Cole et al. 2002)

Stellar mass Blue galaxies are absent above ~3x10 10 M Sun Star formation today occurs in low-mass galaxies Kauffmann et al. (2003) Baldry et al. (2004) log M *

Gas Accretion Halo mass scale constant with time, ~2x10 11 M Sun. 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~10 9 M Sun 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 Vollmer 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

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 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>M v >-20 Giants M v <-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 Dwarfs/Giants

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 Dwarfs/Giants

Observed galaxy clusters Observed field galaxies Bower et al. (2006) model predictions Gilbank & Balogh (2008) Redshift Red Dwarfs/Giants 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 Dwarfs/Giants The evolution in the field can be explained if faint, red galaxies are produced only in groups with masses greater than M Sun.

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 Dwarfs/Giants

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” Groups Z=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 Fraction with [OII] emission lines

Passive galaxies Spitzer IRAC colours are an excellent tracer of low-levels of activity Wilman et al Spirals E/S0 [8  m]-[3.6  m] colour rest [  m]

Star formation in groups Dusty and/or low-levels of star formation in massive galaxies  Break occurs at ~10 11 M Sun.  Group galaxies still show less activity than field galaxies of the same mass log 10 M stellar /M Sun Infrared Active fractionOptically Active fraction Wilman et al. 2007Balogh et al. 2006

Group morphologies Fraction of disk galaxies McGee et al 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 CNOC2 MGC 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 groups Red: passive spirals Black: normal spirals Green: 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 Dark matter Gas Analytic prediction

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/ )

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 Black: data Red: models