1. How are galaxies influenced by their environment? rachel somerville STScI Predictions & insights from hierarchical models with thanks to Eric Bell the COMBO/GEMS team Risa Wechsler Andrey Kravtsov Sandy Faber

2. what are observations telling us? old wisdom: familiar correlations between galaxy observables color-morphology color-magnitude luminosity-metallicity new wisdom: correlation between intrinsic/physical properties, e.g. stellar mass and star formation rate stellar mass & metallicity new wisdom: many fundamental galaxy properties have bimodal distributions, divided by a critical stellar mass

3. Baldry et al. 2003 color luminosity SDSS: color and magnitude Blanton et al. 2003

4. magnitude Sersic index surface density color Blanton et al. 2003

5. stellar mass and age… ~stellar age Kauffmann et al. 2003, 2004 SDSS stellar mass

6. stellar mass and relative star formation rate relative star formation rate stellar mass Brinchmann et al. 2004

7. what imposes these relationships on galaxies (internal/external)? the old wisdom: morphology-density relation: early type fraction increases with density Butcher-Oemler effect: early/blue fraction decreases with cosmic time the new wisdom: (Hogg et al., Blanton et al., Balogh et al., Kauffmann et al.): structural properties have weak dependence on environment spectro-photometric properties have a stronger dependence on environment -- critical density?

8. morphology-density

9. morphology-density as a function of redshift Smith et al. 2004 low density high density fraction of early types projected density lookback time

10. structure and density Kauffmann et al. 2004 increasing density

11. age and relative star formation and local density increasing N Kauffmann et al. 2004 increasing N

12. star formation and local density Balogh et al. 2003

13. Hogg et al. 2003 luminosity has a strong dependence on local density color has a weaker dependence on local density

14. Balogh et al. 2004

15. fraction of red galaxies increases with density but the mean color of the red and blue distributions changes little with density

16. color magnitude Blakeslee et al. 2003 (ACS GTO team) RDCS1252 z=1.24 the color magnitude relation is in place at z~1 and evolution is consistent with passive

17. red dots: early type blue dots: late type rest U-V color rest V magnitude (luminosity) age = 8.4Gyr age = 5.5 Gyr Bell et al. 2003 …in the field as well as clusters (COMBO-17/ GEMS)

18. time  do hierarchical models predict this behaviour? can they give us any insight into what is going on?

19. Wechsler et al. cluster halo ‘Milky Way’ halo

20. Kauffmann et al. 1999 VIRGO/GIF simulations see also Benson et al. 2001; Springel et al. 2001 hierarchical simulations show a clear correlation between color/morphology and density, in qualitative agreement with observations

21. dependence of mean color and morphological fraction on halo mass Diaferio et al. 2001 color fraction of bulge/disk galaxies log halo mass

22. primordial power spectrum star formation stellar feedback chemical enrichment collisional heating radiative cooling merger treestellar populations dust absorption & emission galaxyobservables inflation

23. specific model ingredients reheated gas ejected if V c >150 km/s major mergers (>4:1) trigger bursts of star formation Bruzual & Charlot 2003 multi-metallicity stellar population models

24. SDSS & 2MASS luminosity functions number per unit volume magnitude u-bandg-bandr-band i-bandz-bandK-band (observed LF from Bell et al.

25. luminosity functions by morphology disk dominated bulge dominated

26. gas fraction log stellar mass SDSS Bell et al.

27. gas fraction distributions increasing stellar mass -->

28. color-magnitude relation r-band magnitude

29. color histograms bright--> faint g-r color

30. color of a passively evolving burst formed at z=5 u-r g-r Z=2xsolar Z=solar

31. Global star formation history

32. weighing galaxies with their SED optical color correlated with optical mass-to-light ratio calibrated with dynamical mass measurements (Bell & de Jong; Bernardi et al.) Bell et al. 2003see also Kauffmann et al. 2003

33. stellar mass assembly history rss et al. 2004 new observational estimates from COMBO-17 and GOODS

34. stellar mass assembly history rss et al. 2004 estimates from Glazebrook et al. (GDDS) Rudnick et al. (FIRES) Dickinson et al. (HDFN) Fontana et al. (K20)

35. red: n>2 blue: n<2 red: B/T>0.5 blue: B/T<0.5 color-magnitude and morphology at high redshift

36. K AB <22 13.5 5.8 3.2 1.0 0.5 0.1 rss et al. 2004 GOODS ApJL GOODS missing EROs

37. status: low redshift hierarchical models can be made to reproduce global luminosity/stellar mass distributions at low redshift but don’t produce enough luminous red galaxies color magnitude relation has correct slope (well, sort of) but distributions do not match data and are not bimodal

38. status: high redshift hierarchical models produce enough massive galaxies to z~2 but, do not produce enough red galaxies the mean stellar ages of the massive galaxies are old enough -- color problem is caused by ‘frosting’ of young stars

39. what makes red galaxies red? need a process that quenches star formation in the most massive galaxies without drastically altering the mass assembly/star formation history environment: ram pressure or tidal stripping, harassment? internal: SN or AGN driven wind? global instability?

40. neighbor counts (R=2 Mpc) L=114 Mpc m p =3x10 8 M_sun r_force=1.5 kpc

41. number of neighbors in 2 Mpc spheres cyan: 0-1 blue: 2-3 green: 4-6 rust: 7-11 red: >11

42. number of neighbors vs. halo mass log(1+N) log halo mass

43. color-magnitude by density g-r r-band mag

44. Kauffmann et al. 2004 N=0-1N>17

45. increasing density--> decreasing luminosity --> N=0-1N=2-3N=4-6 N=7-11N>11 -22.5 -21.5 -20.5 -19.5 -18.5

46. Balogh et al. 2004 increasing density--> decreasing luminosity--> u-r

47. relative SFR vs. mass log stellar mass log(SFR/stellar mass)

48. star formation/age as function of local density increasing N Kauffmann et al. 2004 increasing N

49. where does this leave us? where do galaxies become red? simulations: massive halos real universe: all environments (though more often in dense ones) how do galaxies become red? simulations: whole distribution shifts real universe: galaxies ‘hop’ from one distribution to the other why do galaxies become red? simulations: strangulation real universe: ???

50. internal processes all gas driven out of galaxy after a merger by SN or AGN winds? has almost no effect on colors because of continuous infall of fresh gas star formation ‘turned off’ when a bulge (BH?) has grown beyond a critical mass?

51. g-r color r-band magnitude star formation shut off when m_bulge > 2 x 10 10 M_sun better!

52. color distributions bright faint g-r color bimodal!