Color-Magnitude Distribution of Face-on Nearby Galaxies in SDSS DR7
  We have analyzed the distributions in the color-magnitude diagram (CMD) of a
large sample of face-on galaxies to minimize the effect of dust extinctions on
galaxy color. About 300 thousand galaxies with $log(a/b) < $ 0.2 and redshift
$z < 0.2$ are selected from the SDSS DR7 catalog. Two methods are employed to
investigate the distributions of galaxies in the CMD including 1-D Gaussian
fitting to the distributions in individual magnitude bins and 2-D Gaussian
mixture model (GMM) fitting to galaxies as a whole. We find that in the 1-D
fitting only two Gaussians are not enough to fit galaxies with the excess
present between the blue cloud and the red sequence. The fitting to this excess
defines the centre of the green-valley in the local universe to be $(u-r)_{0.1}
= -0.121M_{r,0.1}-0.061$. The fraction of blue cloud and red sequence galaxies
turns over around $M_{r,0.1} \sim -20.1$ mag, corresponding to stellar mass of
$3\times10^{10}M_\odot$. For the 2-D GMM fitting, a total of four Gaussians are
required, one for the blue cloud, one for the red sequence and the additional
two for the green valley. The fact that two Gaussians are needed to describe
the distributions of galaxies in the green valley is consistent with some
models that argue for two different evolutionary paths from the blue cloud to
the red sequence.
