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

We present results of an age and metallicity gradient analysis inferred from both optical and near-infrared surface photometry. The analysis is based on a sample of 36 nearby early-type galaxies, obtained from the Early Data Release of the Sloan Digital Sky Survey and the Two Micron All Sky Survey. Surface brightness profiles were derived in each band and used to study the color gradients of the galaxies. Using simple stellar population models with both optical and near-infrared colors, we may interpret the color gradients in terms of age and metallicity gradients of galaxies. Using g(Z) equivalent to d log Z(met)/d log R and g(A) = d log (age)/d log R to represent the metallicity and age gradients, we found a median value of g(Z) = -0.25 +/- 0.03 for the metallicity gradient, with a dispersion sigma(gZ) = 0.19 +/- 0.02. The corresponding values for the age gradients were g(A) = 0.02 +/- 0.04 and sigma(gA) = 0.25 +/- 0.03. These results are in good agreement with recent observational results, as well as with recent simulations that suggest that both monolithic collapse and major mergers have played important roles in the formation of early-type galaxies. Our results demonstrate the potential of using multi-wave band colors obtained from current and future optical and infrared surveys in constraining the age and metallicity gradients of early-type galaxies.