Color gradients of spiral disks in the Sloan Digital Sky Survey

We investigate the radial color gradients of galactic disks using a sample of ∼ 20 000 face-on spiral galaxies selected from the fourth data release of the Sloan Digital Sky Survey (SDSS-DR4). We combine galaxies with similar concentrations, sizes and luminosities to construct composite galaxies, and then measure their color profiles by stacking the azimuthally averaged radial color profiles of all the member galaxies. Except for the smallest galaxies (R50 < 3 kpc), almost all galaxies show negative disk color gradients with mean g - r gradient Ggr = -0.006 mag kpc-1 and r - z gradient Grz = -0.018 mag kpc-1. The disk color gradients are independent of the morphological types of galaxies and strongly dependent on the disk surface brightness μd, with lower surface brightness galactic disks having steeper color gradients. We quantify the intrinsic correlation between color gradients and surface brightness as Ggr = -0.011 μd + 0.233 and Grz = -0.015 μd + 0.324. These quantified correlations provide tight observational constraints on the formation and evolution models of spiral galaxies.