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

The color bimodal distribution of Galaxies

Li et al. 2006

Separate galaxies into red and blue population

Yang et al. 2008

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. 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

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. 2009

The distribution of other population dwarfs

The nearest neighbor dependent

Wang et al. 2009

The red dwarfs near more massive halos have stronger radial dependence

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

The distribution of red dwarf galaxies

Ludlow 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)