Presentation on theme: "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."— Presentation transcript:
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.
The color bimodal distribution of Galaxies Baldry et al. 2004
The color bimodal distribution of Galaxies
Li et al. 2006
Separate galaxies into red and blue population Yang et al. 2008
The luminosity functions for red and blue galaxies Yang et al. 2009Blanton et al. 2005
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?
Galaxy groups in the SDSS DR4 ( Yang et al ) Sky coverage: 4514 deg^2 Galaxies with redshifts: (408119) Groups selected: (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
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.
The red dwarf galaxies: alternative definition
(1) 20% red dwarfs are not randomly distributed (2) Additional galaxies seem randomly distributed The distribution of red dwarf galaxies Wang et al. 2009
(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
Only the red dwarfs have the radial dependent distribution Wang et al The distribution of other population dwarfs
The nearest neighbor dependent Wang et al The red dwarfs near more massive halos have stronger radial dependence
Test against the N-body simulation Wang et al (1) Case 1: contamination (2) Case 2: randomly distributed (3) Case 3: depends the distance to the nearby more massive halo
The distribution of red dwarf galaxies Ludlow et al. 2008
Red dwarf galaxies in voids Cooper et al. 2008
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)