1. Interpreting the relationship between galaxy luminosity, color, and environment. Andreas Berlind (NYU, CCPP) SPH predictions: Michael Blanton (NYU) David Hogg (NYU) David Weinberg (Ohio State) Daniel Eisenstein (Arizona) Romeel Dave (Arizona) Neal Katz (U. Mass) SDSS Groups: Josh Frieman (Chicago) David Weinberg (Ohio State) Michael Blanton (NYU) David Hogg (NYU) SDSS et al.

2. SPH simulation SPH DM mass density vs. galaxy baryonic mass and age SDSS galaxy density vs. galaxy luminosity and color Ω m =0.4, Ω Λ =0.6, Ω b =0.02h -2, h=0.65, n=0.95, σ 8 =0.8 144 3 dm + 144 3 gas particles, l=50 Mpc/h, m b =8.5x10 8 M sun Gravity + gas dynamics, radiative + Compton cooling, photoionization heating, star formation + feedback Davé, Katz, & Weinberg Berlind et al. (2004)

3. DM halos, central and satellite galaxies The density around galaxies must be correlated with the mass of their underlying DM halo.To what extent is the relationship between galaxy properties and environment simply reflecting their relationship with DM halo mass? Theoretical models predict different properties for central and satellite galaxies in DM halos. This is expected since galaxies undergo different physical mechanisms once they merge into larger systems (e.g., dynamical friction and tidal stripping of mass/stars that can cause destruction, as well as ram pressure stripping of gas that can affect star formation). Also, BCGs observed in clusters suggest separate populations for central/satellite galaxies. halo: ρ/ρ 0 ~200

4. SPH simulation: central and satellite galaxies in DM halos SPH mean halo multiplicity vs. galaxy baryonic mass and age central galaxies all galaxies 1. local galaxy density around galaxies traces the mass of their underlying DM halos. 2. environment dependence for luminous red galaxies reflects the trends for central galaxies in cluster-sized halos. 3. environment dependence for lower luminosity red galaxies reflects the changing mixture of satellite galaxies in cluster- sized halos and central galaxies in low mass halos. 4. environment dependence for blue galaxies reflects the trends for satellite galaxies in group-sized halos. Berlind et al. (2004) Central galaxies are those closest to their DM halos’ most bound particle. satellite galaxies

5. Interpretation Central galaxies Luminosity/mass is strongly correlated with mass of DM halo. Central galaxy is built up via galaxy mergers as DM halo grows. Color/age is weakly correlated with mass of DM halo. Halo formation time and mass accretion history distribution is broad for given mass. Satellite galaxies Luminosity/mass is weakly correlated with mass of DM halo. Most star formation of satellite galaxy probably occurred when it was a central galaxy in its own lower mass halo before it merged. Color/age is strongly correlated with mass of DM halo. Galaxies that form in higher density regions both form earlier and end up in higher mass halos. Also, satellites’ star formation likely shuts off once they merge into larger halos and galaxies that merge into larger mass halos can survive longer since dynamical friction timescales are longer; satellites in lower mass halos must have merged recently.

6. SDSS NYU LSS “sample12” ~2,400 square degrees ~150,000 galaxies r' < 18.5 mag Blanton et al.

7. Equatorial Slice

8. 0.015 < z < 0.1 M r < -20 N = 37,885 n = 0.0057 h 3 Mpc -3 Volume-limited samples 0.015 < z < 0.043 M r < -18 N = 12,067 n = 0.022 h 3 Mpc -3

9. SDSS Group Identification Identify groups using: Friends-of-friends algorithm Different linking length in tangential and line-of-sight directions. b perp =0.2, b par =0.4 Group catalog: 2,627 galaxy groups (N>2) 3,298 galaxy pairs (N=2) 15,637 isolated galaxies SDSS sample: M r <-20 volume limited 37,885 galaxies with 0.015 < z < 0.1

10. Central and satellite galaxies in groups Berlind et al. (2003 ) Impossible to correctly identify central galaxy in each group. SPH simulation + SA models  central galaxy is usually most massive/luminous in halo. Tag most luminous galaxy in each group as “central”. Use group multiplicity as environment measure.

11. Central and satellite galaxies in groups Most luminous galaxies in groups (centrals) Other galaxies in groups (satellites) All galaxies

12. Mean group multiplicity vs. luminosity

13. Mean group multiplicity vs. color

14. Conclusions The relationship between galaxy luminosity, color and environment can be understood from the different relationships for central and satellite galaxies in their DM halos. 1.The environment dependence for luminous red galaxies reflects the trends for central galaxies in cluster-sized halos. ( central galaxy mass/luminosity tightly correlated with halo mass) 2.The environment dependence for lower luminosity red galaxies reflects the changing mixture of satellite galaxies in cluster-sized halos and central galaxies in low mass halos. ( satellite fraction decreasing function of red galaxy luminosity) 3. The environment dependence for blue galaxies reflects the trends for satellite galaxies in group-sized halos. ( satellite galaxy age/color tightly correlated with halo mass) This interpretation of the luminosity-color-environment relation is predicted theoretically by galaxy formation models and confirmed observationally by a study of galaxy groups in the SDSS.