The luminosity density of red galaxies

A complete sample of 7.7 × 10⁴ galaxies with five-band imaging and spectroscopic redshifts from the Sloan Digital Sky Survey is used to determine the fraction of the optical luminosity density of the local universe (redshifts 0.02 < z < 0.22) emitted by red galaxies. The distribution in the space of rest-frame color, central surface brightness, and concentration is shown to be highly clustered and bimodal; galaxies fall primarily into one of two distinct classes. One class is red, concentrated, and high in surface brightness; the other is bluer, less concentrated, and lower in central surface brightness. Elliptical and bulge-dominated galaxies preferentially belong to the red class. Even with a very restrictive definition of the red class that includes limits on color, surface brightness, and concentration, the class comprises roughly one-fifth of the number density of galaxies more luminous than 0.05L* and produces two-fifths of the total cosmic galaxy luminosity density at 0.7 μm. The natural interpretation is that a large fraction of the stellar mass density of the local universe is in very old stellar populations.