The Color Gradient Of The Disk Galaxies In The Sdss

A large sample which contains 6878 face-on disk galaxies is selected from the data released by Sloan Digital Sky Servey (SDSS. DR2). All galaxies in the sample have accurate surface photometry in five optical pass-bands (u.g.i.r. z) and spectral observation. We investigate their radial color gradients and the correlation between the color gradient and the basic parameters, for example, the size, luminosity (stellar mass), surface brightness (surface mass density), and concentration parameter.We also select a sub-sample whose galaxies are brighter than 15mag. and fit the radial surface brightness profile of these galaxies using two component (bulge + disk) model. Then we obtain many important parameters, such as. the ratio of bulge to disk luminosity (B/D). the effective radius of bulge (R_B) and the half-light radius of disk component (R_d). We find that (R_d) obtained from the model fitting is equal to the observed half-light radius (R₅₀) of the galaxy. We also find that B/D at R₅₀ is smaller than 0.1 for most of galaxies. So we separate the galaxy into two part at R₅₀, the inner part is the bulge component while the outer part is dominated by disk component. We mainly investigate the color gradients of outer parts of the galaxies in the following work.Because the surface photometry is seriously affected by the sky light at large radius, so the signal-to-noise ratio (S/N) of surface brightness is low. To resolve this problem, we construct the "composite galaxy' using a group of similar galaxies, and fit the color profile of the "composite galaxy" to obtain the radial color gradient. We find that most of spiral galaxies have negative color gradients, that is, the outer, the bluer color is.We also investigate the relationship between the color gradient and the basic parameters of galaxy and find that the larger, more luminosity and lower surface brightness galaxies tend to have sloper gradients. However, the color gradient has a stronger relationship with the size and surface brightness than that with the luminosity. We also find that the bulge of spiral galaxy has little relationship with the color gradient of disk component.Considering the color gradient is mainly caused by the age gradient, the colorgradient of disk component, is directly reflect the star formation history (SFH) of disk. According to our conclusion, the angler momentum (size) and surface mass density of spiral galaxy are the main factors which influence the SFH of disk component while the mass is less impotent and the bulge does not effect the SFH of disk at all. These conclusions provide farther constrains to the formation and evolution model of disk of spiral galaxies.