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The important role of galaxy groups

The important role of galaxy groups. Michael Balogh Department 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.

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The important role of galaxy groups

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  1. The important role of galaxy groups Michael Balogh Department of Physics and Astronomy University of Waterloo

  2. Galaxy Clusters • A standard picture to motivate environmental effects: • Clusters are dominated by bright, red ellipticals

  3. 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

  4. 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

  5. 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

  6. 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.

  7. Ha for Virgo galaxy Ha for normal galaxy Direct evidence of environmental effects • Ram-pressure stripping in Virgo Kenney et al. 2003 Vollmer et al. 2004 • Truncated Ha disks in cluster spirals • These galaxies have -20<MB<-18 Koopmann & Kenney 2004 also: Vogt et al. 2004 • Passive Spirals • S0, dSph, UCDs • Wolf’s dusty spirals? Peak in infall region?

  8. 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! • 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 Dekel & Birnboim 2006

  9. Isolated Galaxies • No low-mass, red galaxies, as predicted Increasing stellar mass Red sequence threshold Balogh, Salton et al. in prep.

  10. Predictions • SFH of central galaxy depends on halo mass • SFH of satellite galaxies independent of halo mass for M>1012MSun • Note: satellites are red even if they have low mass Croton et al. 2006 models; from Weinmann et al. 2006

  11. Fraction of Late-type galaxies Groups and clusters • 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

  12. 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

  13. Red Galaxy luminosity function • Dwarfs: -18.2>Mv>-20 • Giants Mv<-20 Weinmann et al. haloes (SDSS) Courtesy: Sean McGee Red De Lucia et al. (2007)

  14. VVDS: Zucca et al. 2006 Driver et al. 2006 Marzke & Da Costa 1997 COMBO-17: Bell et al. 2004 PISCES: Tanaka et al. 2006 Red Dwarf/Giant Ratio Field surveys • Faint red galaxies build up with time. • Note: these “dwarfs” are MV~-19

  15. Stott et al. 2007 VVDS: Zucca et al. 2006 Lu et al. 2007 Driver et al. 2006 Marzke & Da Costa 1997 De Lucia et al. 2007 Tanaka et al. 2007 COMBO-17: Bell et al. 2004 PISCES: Tanaka et al. 2006 Red Dwarf/Giant Ratio Clusters • Local clusters have more red dwarfs than the field. • Clusters evolve strongly: faint red population appears at z<1 ? Gilbank et al. 2007

  16. Stott et al. 2007 VVDS: Zucca et al. 2006 Lu et al. 2007 Driver et al. 2006 Marzke & Da Costa 1997 De Lucia et al. 2007 Tanaka et al. 2007 COMBO-17: Bell et al. 2004 PISCES: Tanaka et al. 2006 Red Dwarf/Giant Ratio Bower et al. 2006 (clusters) • Models get local clusters right! Gilbank et al. 2007

  17. Stott et al. 2007 VVDS: Zucca et al. 2006 Lu et al. 2007 Driver et al. 2006 Marzke & Da Costa 1997 De Lucia et al. 2007 Tanaka et al. 2007 COMBO-17: Bell et al. 2004 PISCES: Tanaka et al. 2006 Red Dwarf/Giant Ratio • Oops. Too many faint red galaxies in the field! Bower et al. 2006 (field) Gilbank et al. 2007

  18. Red Dwarf/Giant Ratio Models • Models predict little evolution • Strangulation is too effective in small groups, which are dominant at z<1 Clusters Field

  19. Two solutions to the faint red galaxy problem • 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. • Timescale? • Maybe it’s still strangulation, operating in smaller haloes, but more slowly?

  20. 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.

  21. CNOC2:Groups at z~0.4 • ~200 groups between z~0.1 and z~0.55 Millennium Simulation All haloes “CNOC2” Groups Z=0.5 • 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 McGee et al. 2007

  22. Evolution in Groups • SFH of galaxies in groups are similar to the field, and evolve with it Wilman et al. 2005

  23. Groups @ z=0.5 • Active fraction weakly depressed relative to field • No evidence for dramatic effects. • Models predict much lower fractions Bower et al. model groups Balogh et al. 2006

  24. 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. Black: data Red: models McGee et al. 2007

  25. Two solutions to the faint red galaxy problem • 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. • Timescale? • Maybe it’s still strangulation, operating in smaller haloes, but more slowly?

  26. 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

  27. 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 McCarthy et al. 2007

  28. Hot stripping in clusters • Onset of stripping is delayed • a=2, b=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 McCarthy et al. 2007

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

  30. 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.

  31. Extra slides

  32. 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)

  33. 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

  34. Threshold mass • Difficult to match observations if all SF shuts off above ~1012 MSun SDSS clusters 1012 MSun threshold Poggianti et al. 2006

  35. Threshold mass • Difficult to match observations if all SF shuts off above ~1012 MSun SDSS clusters 1014 MSun threshold Poggianti et al. 2006

  36. Groups: Model predictions • Bower et al. (2006) models predict groups should be more like clusters • Too large a difference compared with the field Field Group mean

  37. 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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