Quantifying the bimodal color-magnitude distribution of galaxies

Baldry et al. 2004, published in ApJ, Vol. 600, pp. 681-694 (link: ADS).

Abstract

We analyse the bivariate distribution, in color versus absolute magnitude (u-r vs. M_r), of a low redshift sample of galaxies from the Sloan Digital Sky Survey (SDSS; 2400 deg^2, 0.004 < z < 0.08, -23.5 < M_r < -15.5). We trace the bimodality of the distribution from luminous to faint galaxies by fitting double-Gaussians to the color functions separated in absolute magnitude bins. Color-magnitude (CM) relations are obtained for red and blue distributions (early- and late-type, predominantly field, galaxies) without using any cut in morphology. Instead, the analysis is based on the assumption of normal Gaussian distributions in color. We find that the CM relations are well fit by a straight line plus a tanh function. Both relations can be described by a shallow CM trend (slopes of about -0.04, -0.05) plus a steeper transition in the average galaxy properties over about two magnitudes. The midpoints of the transitions (M_r=-19.8 and -20.8 for the red and blue distributions, respectively) occur around 2x10^10 M_solar after converting luminosities to stellar mass. Separate luminosity functions are obtained for the two distributions. The red distribution has a more luminous characteristic magnitude and a shallower faint-end slope (M^*=-21.5, alpha=-0.8) compared to the blue distribution (alpha=-1.3 depending on the parameterization). These are approximately converted to galaxy stellar mass functions. The red distribution galaxies have a higher number density per magnitude for masses greater than about 3x10^10 M_solar. Using a simple merger model, we show that the differences between the two functions are consistent with the red distribution being formed from major galaxy mergers.

Data:

  • Double Gaussian fitting parameters, e.g. as per Figures 5-7 (Note however that these data have been updated to DR4 and a different method is used. Galaxies with high / low concentrations are given higher weight when fitting the color mean and color dispersions of the red / blue sequences. This is described in Baldry et al. 2006 Sec 3.1).
  • Tanh and Schechter parameters for the sequences (as above).
  • The information in the above two data files is also presented in the figure. The symbols and error bars represent the Gaussian parameters while the dotted lines represent the tanh and Schechter fits. The green lines show the best-fit dividing line. The last panel represents a reconstruction of the color-magnitude distributions.
  • Color-magnitude 2D histogram as per Figure 2.
    		• colour magnitude, dispersion, luminosity functions

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