Galaxy groups Driving galaxy evolution since z=1 Michael Balogh Department of Physics and Astronomy University of Waterloo

Outline 1.What we know about galaxy formation a)The local Universe b)Evolution since z<1 2.GEEC: Groups at 0.3<z<0.5 3.Model development

We observe starlight – which results from the condensation of baryonic matter

Matter and Energy Baryons make up less than 5% of the matter and energy in the Universe Spergel et. al 2003, 2006

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

The cooling catastrophe Cooling occurs primarily through bremsstrahlung radiation, so t cool  T 1/2  -1 The typical density of haloes is higher at early times:   (1+z) 3 Thus, gas cools very efficiently in small haloes at high redshift.

 >95% of baryons are dark The inefficiency of star formation  stars = ±  stars /  baryon =0.03

Why so few stars? Simulation: dark matter in the Local Group Overcooling leads to the formation of hundreds more small galaxies than are observed. Dark matter Stars

Stellar mass Salim et al Blue galaxies are absent above ~3x10 10 M Sun Star formation today occurs in low-mass galaxies From GALEX & SDSS data

Stellar mass Most star formation today occurs in M=10.5 galaxies. Why? Gilbank et al SFR Density

Low-mass galaxies Low masses: photoionization and supernovae reduce SFR

Massive galaxies Halo mass scale constant with time, ~2x10 11 M Sun. Separates “hot” and “cold” accretion (e.g. White & Frenk 1991) Dekel & Birnboim 2006

Massive galaxies Outflows from massive black hole accretion can provide up to 6x10 54 J AGN feedback helps eliminate bright blue galaxies (Springel et al. 2005; Croton et al. 2006; Bower et al. 2006)

Galaxy Clusters Clusters are characterised by bright, red ellipticals

equally strong dependence on halo mass and stellar mass Even low-mass galaxies in clusters are mostly passive Kimm et al (SDSS data) The role of environment: halo mass

Evolution

Evolution I Gradual reduction in SFR at all masses, among “active” population Noeske et al “Star-forming” galaxies in the AEGIS survey

Evolution II Plus growth of “red-and-dead” galaxies, starting with the most massive zCOSMOS: Pozzetti et al <z<1 z=0 0.1<z<0.35

Group evolution in zCOSMOS Evolution is more advanced in clusters Restricted to massive galaxies Iovino et al F passive Groups Field z=0

GEEC Group Environment Evolution Collaboration Michael Balogh, Sean McGee (Waterloo) Richard Bower (Durham) John Mulchaey, Gus Oemler (Carnegie) Dave Wilman, Jen Connelly, Alexis Finoguenov (MPE) Laura Parker, Annie Hou (McMaster)

Groups at 0.3<z<0.5 ~200 groups between z~0.1 and z~0.55 Selected from CNOC2 survey 26 groups 0.3<z<0.55 followed-up at Magellan IRAC and MIPS 3 Orbit GALEX Deep Chandra/XMM HST ACS (1 orbit in F775W) for 20 groups GEEC Groups 0.3<z<0.5 Millennium Simulation All haloes McGee et al. 2007

GEEC: GALEX data

SED fits

Star-forming group galaxies SSFR-M correlation independent of environment GEEC McGee et al. in prep

Groups at z=0.5 There are more galaxies in groups without any star formation Note that most of the lowest- mass galaxies are still actively forming stars zCOSMOS limit GEEC McGee et al. in prep

SFR evolution Low-redshift comparison sample from SDSS Field in good agreement with Noeske et al. Group environments identical to field. McGee et al. in prep

Rapid evolution in groups Groups have been diverging from the field since z=0.4 McGee et al. in prep

Group evolution Field galaxies evolve slowly: SSFR decreases steadily with time. In addition to this, star formation is shut off in group galaxies, accelerating their evolution.

Models

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

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); see also Gilbank & Balogh (2008) Model predictions Star-forming fraction

Rapid strangulation Compare GEEC group galaxy colour distribution with models 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, hot stripping Idealised simulations Takes ~2 Gyr to remove half the gas mass  Still plenty of hot fuel left 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.

Slow strangulation Models which slow the rate of transformation  Destroys distinct bimodality  Is the problem with the strangulation – or with the normal feedback cycles? Balogh et al. (2009)

Conclusions/Future Directions Groups accelerate the termination of star formation at z<1  For reasons that are still not understood. GEEC2: Sample of 20 groups at z=1 selected from zCOSMOS  X-ray detected from very deep Chandra/XMM images  Gemini spectroscopy proposed to return ~15-20 members per group  HST data

Extra slides

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) z=0 z=0.5 z=0.8

Cluster growth via groups McGee et al. (2009) Clusters grow via:  Major mergers between clusters  Accretion of groups  Accretion of isolated galaxies Scatter in cluster properties can be a good tracer of group preprocessing (Balogh et al. 2009)

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

Timescales Starvation model seems a good fit to the passive spirals in GEEC McGee et al. in prep GEEC groups Red: passive spirals Black: normal spirals