The influence of environment on galaxy populations

1. The influence of environment on galaxy populations Michael Balogh University of Waterloo, Canada

2. Outline • Low redshift • Simple trends encompass most of what we know of as environmental influences • Models: what works and what doesn’t • Redshift evolution • The future: what’s next?

3. The influence of environment on galaxy populations Populations • Current star formation rate • Recent star formation • Stellar mass (average SFR) • Morphology (of stars, neutral gas, ionized gas) • AGN • Gas content Environment • Mass of dark matter halo • Position within halo • Local density • Large-scale density

4. The influence of environment on galaxy populations • Nature vs. nurture? • Entangled in current models • Gas accretion, merger, and feedback history scale with halo mass. • No longer the right question? • A better question: what physics operates in haloes of a given mass, at a given epoch? • Today’s population is the result of different environments at different epochs: cannot try to isolate one mechanism as responsible for the observed trends.

5. The local Universe

6. Colour-magnitude distribution • Nearby galaxies seem to fall into two surprisingly well-defined, smoothly varying distributions. • Colour, luminosity, concentration, star formation rate Blanton et al. 2004

7. Bright Faint (u-r) Colour-magnitude distribution • Colour distribution in 0.5 mag bins can be fit with two Gaussians • Mean and dispersion of each distribution depends strongly on luminosity • Dispersion includes variation in dust, metallicity, SF history, and photometric errors • At bright magnitudes, significant fraction of “blue” population “contaminates” red: c.f. talk by Wolf. Baldry et al. 2003

8. Fraction of red galaxies depends strongly on density. This is the primary influence of environment on the colour distribution. • Mean colours depend weakly on environment: transitions between two populations must be rapid (or rare at the present day) Balogh et al. 2004

9. Fraction of red galaxies depends strongly on density. This is the primary influence of environment on the colour distribution. • Mean colours depend weakly on environment: transitions between two populations must be rapid (or rare at the present day) • Trend is not completely absent for fainter galaxies; but never dominant Balogh et al. 2004

10. The star-forming population • Carter et al. (2001) • 3150 nearby galaxies • Ha for SF galaxies does not depend on environment • Triggering of SF occurs on small spatial scales • Rines et al. 2005: Ha distribution in virial, infall and field regions nearly identical. • Hard to explain with simple, slow-decay models (e.g. Balogh et al. 2000)

11. [-22,-23] [-21,-22] [-20,-21] [-19,-20] [-18,-19] Halo mass dependence R luminosity • Environment: halo mass • Use luminosity as tracer of mass. Compare with theoretical mass function • At fixed mass the late-fraction depends weakly on luminosity • Late-type fraction depends most strongly on halo mass Weinmann et al. 2005

12. [-22,-23] [-21,-22] [-20,-21] [-19,-20] [-18,-19] colour SFR concentration Halo mass dependence R luminosity • Average properties of galaxies in either peak is independent of halo mass • But depends on luminosity Weinmann et al. 2005

13. Local effects? • Still a (weak) trend with radius in haloes of fixed mass • Dependence on luminosity (surprisingly?) weak 1014<M<1015 1013<M<1014 Weinmann et al. 2005

14. Conformity • Properties of “satellite” galaxies appear to be connected with properties of “central” (actually brightest) galaxy Weinmann et al. 2005 Similar to effect seen in 2PIGG groups? See Vince Eke’s talk. Definition of central?

15. Implications • Simple dependence of “late-type” fraction on environment characterizes much of observed trends (e.g. SFR-density, morphology-density, colour-density etc.). • Interpretation? • Two modes of formation. Within each peak is variance due to dust, metallicity (second-order effects). • Transitions: Where do S0, E+A fit in? • Burst vs. continuous SFR (Kauffmann et al. 2005)

16. Ha for Virgo galaxy Ha for normal galaxy Signs of Nurture: Virgo spirals • Ram-pressure stripping in Virgo Kenney et al. 2003 Vollmer et al. 2004 • Truncated Ha disks in clusters Koopmann & Kenney 2004 also: Vogt et al. 2004

17. Signs of Nurture: morphology and SFR • Passive Spirals • E+A galaxies? • S0, dSph, UCDs • Wolf’s dusty spirals? Peak in infall region? • e.g. Christlein & Zabludoff (2005) • Residual [OII] after subtracting expectation for given B/T, D4000 and Mstar. • SFR gradient is not entirely: • Consequence of MDR • Consequence of change in mass function • Effect of initial conditions

18. AGN • AGN fraction independent of density • Surprising? Miller et al. (2003) Carter et al. (2001)

19. Models

20. Semi-analytic approach • Trace merger histories with N-body simulations (cannot use Press-Schechter because you need to know where the galaxies are) • More massive haloes form earlier: longer merger history. • There is also a larger-scale bias: haloes of a given mass form earlier in denser environments (Sheth & Tormen 2004; Abbas & Sheth 2005; Harker et al. 2005) • Make simple assumptions about gas accretion (e.g. no accretion onto satellites) and feedback (supernova, AGN)

21. General trends: successes Okamoto & Nagashima (2003) SFR-radius Springel et al. 2001: morphology-density relation 0.0 0.5 1.0 1.5 2.0 R/R200 Diaferio et al. (2001) colour-radius

22. cluster Spirals Ellipticals All -24 -22 -20 -18 -16 MV-logh -24 -22 -20 -18 -16 MV-logh Bimodality? Cole et al. 2000 Supernova feedback prescription does not produce bimodal colour distribution at faint magnitudes. • Springel et al. 2001; Diaferio et al. 2001 • Bimodality in field not clear • All cluster galaxies are red Data Model Okamoto & Nagashima 2003 • SFR is suppressed in all galaxies: blue peak is distorted

23. Keres et al. (2005): SPH simulations reproduce trend of decreasing SFR with increasing density (see also Berlind et al. 2004). Confirm this is due to reduced accretion of hot gas SPH simulations But colour-distribution of galaxies doesn’t look quite right… SPH SFR Hot accretion Observed Cold accretion

24. BH accretion rate Magorrian-AGN No feedback Improving the colour distribution • Springel, Di Matteo & Hernquist (2005) • Including black hole feedback terminates star formation more quickly. Leads to rapid reddening of merger remnants • Sijacki & Springel 2005 • AGN feedback removes young population in cD galaxies

25. Improving the colour distribution • Croton et al. (2005) • Radio-feedback most efficient in large groups. • Proportional to Mgas×MBH Cooling rate (Msun/yr)

26. Models: summary • When feedback parameters are tuned to reproduce the field luminosity function and colour distribution, what will we find as a function of environment? • General trends will be reproduced. But will it be for the right reasons? • Any differences in detail: will they signify “nurture” processes? Or just that feedback parameters need further tuning?

27. Back to observations: Evolution

28. Evolution: clusters(briefly) • Morphology-density relation (see talks by Postman, Dressler) • Fewer S0 in z=1 clusters, but non-zero • Little evolution in MDR z=1 to z=0.5 • Suggests high-z MDR is primordial, with z<0.5 environment-driven evolution • SFR and colour gradients • Radial gradients steeper in the past (Ellingson et al. 2001; Kodama & Bower 2001) • Can be related to truncation of star formation in an infalling field population

29. Clusters • Tanaka et al. 2005 (see poster) • tight CMR in place in clusters to z=0.8 • Faint end of CMR in groups formed z~0.5 • No CMR in field at z=0.8 • Also De Lucia (2004): faint end of red sequence disappears at z>0.5

30. Nakata et al. 2005 Field Postman, Lubin & Oke 2001 van Dokkum et al. 2000 2dF Fisher et al. 1998 Czoske et al. 2001 Clusters Cluster galaxy evolution • Supported by observed evolution in [OII]-emission fraction (Nakata et al. 2005) • Field evolves much more strongly than clusters (for bright galaxies)

31. Evolution: photo-z surveys • Similar rate of increase in red fraction in the field and clusters • average field red sequence galaxy came into the sample later Red galaxy fraction High density All galaxies Red galaxy fraction 0 0.2 0.4 0.6 0.8 1.0 0.2 0.4 0.6 0.8 Low density MV < -20 Redshift COMBO-17: E. Bell et al. CFHTLS: Nuijten et al. (2005)

32. Luminosity, density and redshift dependence of red fraction SDSS z=0: Balogh et al. (2004) RCS z>0: Yee et al. (2005)

33. Luminosity, density and redshift dependence of colour RCS z>0: Yee et al. (2005) SDSS z=0: Balogh et al. (2004)

34. Luminosity, density and redshift dependence of colour • Little evolution in red peak colour RCS z>0: Yee et al. (2005) SDSS z=0: Balogh et al. (2004)

35. Luminosity, density and redshift dependence of colour • Little evolution in red peak colour • Colours of bright blue galaxies evolve strongly RCS z>0: Yee et al. (2005) SDSS z=0: Balogh et al. (2004)

36. Galaxy groups at z=0.4 • Selected from CNOC2 survey • >30 nights Magellan spectroscopy (better completeness, depth) • ACS image of ~30 groups • GALEX data rolling in slowly • Spitzer (IRAC and shallow MIPS) data from GTO programs • Collaborators: Dave Wilman (MPE), Richard Bower (Durham), Gus Oemler, John Mulchaey (Carnegie), Ray Carlberg (Toronto)

37. Groups at z=0.4: Morphologies E/S0-dominated group s=226 km/s Spiral-dominated group s=270 km/s

38. Morphologies: early results • There are fewer spiral galaxies in groups than in the field, at the same redshift. • No evidence for more disturbance/irregularities in group galaxies Groups E/S0 fraction Field Field Spiral fraction Spiral fraction Groups Groups Vel. Dispersion (km/s)

39. Field Groups The connection between star formation rate, morphology and environment Distributions are corrected for differences in luminosity function between group and field S0 Elliptical Early spiral Late spiral Like clusters, groups contain passive spirals: disk morphology but low star formation rates

40. Stellar mass-SFR Rosati? z=1 SDSS (Kauffmann et al.) • Stellar masses from archival Spitzer (IRAC) data • Significant star formation seen in more massive galaxies than locally: downsizing? • No significant difference between group and field for this subsample.

41. Evolution in groups • Use [OII] equivalent width to find fraction of galaxies without significant star formation • most galaxies in groups at z~0.4 have significant star formation – in contrast with local groups • cf. Gonzalez talk: supergroup Fraction of non-SF galaxies Wilman et al. (2004)

42. Group SFR evolution Groups • Fraction of non-SF galaxies increases with redshift • for both groups and field • Insensitive to aperture effects • Evolution cannot be account for by passive-evolution models. Require truncation of star formation (both groups and field) Fraction of non-SF galaxies Field Fraction of non-SF galaxies Wilman et al. 2004

43. Nakata et al. 2005 Field Postman, Lubin & Oke 2001 van Dokkum et al. 2000 2dF Fisher et al. 1998 Czoske et al. 2001 Clusters Group Evolution Groups: Wilman et al. (2005)

44. High redshift • Spectroscopic survey: ~100 redshifts 1.48<z<2.89 • Overdense region has more massive, older galaxies • Consistent with expectations for earlier formation time (1600 Myr vs 800 Myr) Steidel et al. (2005)

45. High redshift • UV-selected LBG survey • No environmental dependence of SFR • Can be consistent: cluster galaxies get head start, but instantaneous SFR the same • Even at z=0 it seems star-forming galaxies have a distribution independent of environment Bouché & Lowenthal (2005)

46. The future • Theory: still has a lot of catching up to do • Thus we are in discovery mode rather than testing mode • Observations: • Dust-obscured SF (Spitzer, Herschel) • AGN/SF connection at z>0 • Lower luminosities • Spatial dependence of SFR (i.e. IFU spectroscopy) • Transitional galaxies