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 term of age and metallicity gradients of galaxies. Using gZ ≡ d logZmet/d logR and gA = d logAge/d logR to represent the metallicity and age gradients, we found a median value of gZ = −0.25 ± 0.03 for the metallicity gradient, with a dispersion σgZ = 0.19± 0.02. The corresponding values for the age gradients were gA = 0.02 ± 0.04 and σgA = 0.25 ± 0.03. These results are in good agreement with recent observational results, as well as with recent simulations that suggest 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-waveband colors obtained from current and future optical and infrared surveys in constraining the age and metallicity gradients of early-type galaxies. Subject headings: galaxies: photometry galaxies: metallicity galaxies: age 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 College of Physical Sciences, Graduate School of the Chinese Academy of Sciences, P.O. Box 3908, Beijing 100039, P. R. China Max-Planck-Institute für Astrophysik, Karl Schwarzschild Str. 1, Postfach 1317, 85741 Garching, Germany