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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 www.nbody.net The halo model Hot baryons ~106 K for galaxies, hence invisible Radiative cooling Dark matter The cooling catastrophe Cooling occurs primarily through bremsstrahlung radiation, so tcool T1/2r-1 The typical density of haloes is higher at early times: r (1+z)3 Thus, gas cools very efficiently in small haloes at high redshift. The inefficiency of star formation Wstars= 0.0014 ± 0.00013 Wstars/ Wbaryon =0.03 >95% of baryons are dark Why so few stars? Simulation: dark matter in the Local Group Dark matter Stars • Overcooling leads to the formation of hundreds more small galaxies than are observed. Stellar mass • Blue galaxies are absent above ~3x1010 MSun • Star formation today occurs in low-mass galaxies From GALEX & SDSS data Salim et al. 2007 Stellar mass SFR Density • Most star formation today occurs in M=10.5 galaxies. Why? Gilbank et al. 2009 Low-mass galaxies • Low masses: photoionization and supernovae reduce SFR Massive galaxies • Halo mass scale constant with time, ~2x1011 MSun. • 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 6x1054 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 The role of environment: halo mass • equally strong dependence on halo mass and stellar mass • Even low-mass galaxies in clusters are mostly passive Kimm et al. 2009 (SDSS data) Evolution Evolution I • Gradual reduction in SFR at all masses, among “active” population “Star-forming” galaxies in the AEGIS survey Noeske et al. 2007 Evolution II z=0 • Plus growth of “red-and-dead” galaxies, starting with the most massive zCOSMOS: Pozzetti et al. 2009 0.75<z<1 0.1<z<0.35 Group evolution in zCOSMOS z=0 Fpassive Groups Field • Evolution is more advanced in clusters • Restricted to massive galaxies Iovino et al. 2009 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 Millennium Simulation All haloes • • • • • • 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 McGee et al. 2007 GEEC: GALEX data SED fits Star-forming group galaxies • SSFR-M correlation independent of environment GEEC 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 lowestmass galaxies are still actively forming stars GEEC GEEC zCOSMOS limit 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 Kenney et al. 2003 Vollmer et al. 2004 • 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 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): M* t 2.2 10 10 M Sun 0.3 Gyr • Low stellar-mass, red galaxies are predicted to be in groups Satellite galaxies at z=0 Star-forming fraction • 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? Model predictions Weinmann et al. (2006); see also Gilbank & Balogh (2008) 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 • z=0 z=0.5 z=0.8 Knobel et al. (2009) Cluster growth via groups • 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) McGee et al. (2009) Group morphologies • 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 Fraction of disk galaxies Fraction of disk galaxies McGee et al. 2007 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 Red: passive spirals Black: normal spirals GEEC groups McGee et al. in prep