Color gradients of spiral disks in the Sloan Digital Sky Survey

We investigate the radial color gradients of galactic disks using a sample of similar to 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 (R-50 < 3 kpc), almost all galaxies show negative disk color gradients with mean g - r gradient <(G)over bar>(gr) = -0.006 mag kpc(-1) and r - z gradient (G) over bar (rz) = -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 mu(d), with lower surface brightness galactic disks having steeper color gradients. We quantify the intrinsic correlation between color gradients and surface brightness as G(gr) = -0.011 mu(d) + 0.2333 and G(rz) = -0.015 mu d + 0.324. These quantified correlations provide tight observational constraints on the formation and evolution models of spiral galaxies.