Optical and infrared color distributions in nearby early-type galaxies and the implied age and metallicity gradients

We analyze the optical and near-infrared (NIR) surface photometries for a sample of 36 nearby early-type galaxies based on the eight broad-band images obtained from the Early Data Release (EDR) of the Sloan Digital Sky Survey (SDSS) and the Two Micron All Sky Survey (2MASS). The surface brightness profiles of most galaxies can be well fitted by the de Vaucouleur law, and the effective radius decreases significantly from blue optical to NIR bands. We also study the color gradients of each galaxy among all these eight bands. When transformed to the standard photometric system, the average color gradients in U−R, B − R and J − KS are −0.21, −0.08 and −0.09, respectively. With the use of simple stellar population (SSP) model, we interpret the observed color gradients in terms of the gradients in the metallicity and age of the stellar components in these galaxies. If age gradient is assumed to be absent, the observed color gradients implies a median metallicity gradient of −0.22, although the scatter is quite big among different galaxies. Fitting both age and metallicity simultaneously, and using gZ = d logZmet/d logR and gA = d logAge/d logR to represent the metallicity and age gradients, we found that the median value gZ = −0.24±0.03 National Astronomical Observatories, CAS, Beijing 100012, P.R. China Shanghai Astronomical Observatory, CAS, Shanghai 200030, P.R. China Astronomy Department, University of Massachusetts, Amherst MA 01003, USA Department of Physics, Tianjin Normal University, Tianjin 300074, P. R. China Graduate School, Chinese Academy of Sciences, Beijing 100080, P. R. China Max-Planck-Institut für Astrophysik, Karl Schwarzschild Str. 1, Postfach 1317, 85741 Garching, Germany