Galaxy bimodality versus stellar mass and environment

Baldry et al. 2006, published in MNRAS, Vol. 373, pp. 469-483 (links: astro-ph; ADS).

Abstract

We analyse a z < 0.1 galaxy sample from the Sloan Digital Sky Survey focusing on the variation of the galaxy colour bimodality with stellar mass and projected neighbour density Sigma, and on measurements of the galaxy stellar mass functions. The characteristic mass increases with environmental density from about 10^10.6 Msun to 10^10.9 Msun (Kroupa IMF, H_0=70) for Sigma in the range 0.1--10 per Mpc^2. The galaxy population naturally divides into a red and blue sequence with the locus of the sequences in colour-mass and colour-concentration index not varying strongly with environment. The fraction of galaxies on the red sequence is determined in bins of 0.2 in log Sigma and log mass (12 x 13 bins). The red fraction f_r generally increases continuously in both Sigma and mass such that there is a unified relation: f_r = F(Sigma,mass). Two simple functions are proposed which provide good fits to the data. These data are compared with analogous quantities in semi-analytical models based on the Millennium N-body simulation: the Bower et al. (2006) and Croton et al. (2006) models that incorporate AGN feedback. Both models predict a strong dependence of the red fraction on stellar mass and environment that is qualitatively similar to the observations. However, a quantitative comparison shows that the Bower et al. model is a significantly better match; this appears to be due to the different treatment of feedback in central galaxies.

Data:

  • Galaxy stellar mass functions for 12 environment bins as per Figure 8(a). (Stellar masses use the Kroupa '01 IMF and H_0=70. Omega_stars is about 0.002. Note on IMF use.)
  • GSMF Schechter fits for 12 environment bins as per Figure 8(b-d).
  • GSMF double Schechter fits as per Figure A1.
  • Color-mass 2D histograms for 6 environment bins as per Figure 9.
  • Fraction of red-sequence galaxies for 13 mass x 12 environment bins as per Figures 11-12.
  • Spectroscopic completeness as per Figure 3.
  • Color-magnitude 2D histogram as per Figure 7(b).
  • DR7 galaxy catalog with Sigma values (6.2M), see this file for column details.

    Figures:

  • Environmental densities around galaxies
  • Bimodality in the colour-mass distribution for different environments

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