1. Xiaohu Yang Shanghai Astronomical Observatory Partner group of MPA Collaborators: Yu Wang (SHAO), H.J. Mo (UMass), F.C. van den Bosch (MPIA), Neal Katz (UMass), A. Pasquali (MPIA), D.H. McIntosh (UMK), S. Weinmann (MPA), etc.

2. The color bimodal distribution of Galaxies Baldry et al. 2004

3. The color bimodal distribution of Galaxies

4. Li et al. 2006

5. Separate galaxies into red and blue population Yang et al. 2008

6. The luminosity functions for red and blue galaxies Yang et al. 2009Blanton et al. 2005

7. The 4500 faintest galaxies With M_r>-17.05, dwarf galaxies There are a significant fraction of red dwarf galaxies. Where are they? What makes them red?

8. Galaxy groups in the SDSS DR4 ( Yang et al. 2007 ) Sky coverage: 4514 deg^2 Galaxies with redshifts: 369447 (408119) Groups selected: 301237 (300049) Galaxies are grouped according to their common halos Halo mass is assigned for each group The most massive galaxy is defined as the central galaxy

9. The environment of red dwarf galaxies Satellite galaxies (45%)  They are in massive halos Central galaxies (55%)  They are in small halos  They are at r_p/r_200 to a nearest more massive halo.

10. The red dwarf galaxies: alternative definition

11. (1) 20% red dwarfs are not randomly distributed (2) Additional galaxies seem randomly distributed The distribution of red dwarf galaxies Wang et al. 2009

12. (1) 20% red w.r.t. the dwarfs of similar stellar masses (2) 20% blue (3) 20% random (4) 20% high concentration What about other population dwarfs? Wang et al. 2009

13. Only the red dwarfs have the radial dependent distribution Wang et al. 2009 The distribution of other population dwarfs

14. The nearest neighbor dependent Wang et al. 2009 The red dwarfs near more massive halos have stronger radial dependence

15. Test against the N-body simulation Wang et al. 2009 (1) Case 1: contamination (2) Case 2: randomly distributed (3) Case 3: depends the distance to the nearby more massive halo

16. The distribution of red dwarf galaxies Ludlow et al. 2008

17. Red dwarf galaxies in voids Cooper et al. 2008

18. Summary and discussion There are about 70% central dwarf galaxies  About ¼ of them are red according to bimodal color distribution These red central dwarf galaxies are not randomly distributed  They are preferentially distributed around massive halos  There is also a background distribution. The dependence does not exist for high concentration dwarf galaxies. The formation of red dwarf galaxies  Massive halo environment, quenches the star formation  Other mechanism that quenches the star formation in small halos (in low density regions)