2004ApJ...612..202M

We study the internal color properties of a morphologically selected sample of spheroidal galaxies taken from the Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) Early Release Observation program of UGC 10214 (``the Tadpole''). By taking advantage of the unprecedented high resolution of the ACS in this very deep data set, we are able to characterize spheroids at subarcsecond scales. Using the V₆₀₆and I₈₁₄ bands, we construct V-I color maps and extract color gradients for a sample of spheroids at I₈₁₄<24 mag. We assess the ability of the ACS to make resolved color studies of galaxies by comparing its results with the multicolor data from the Hubble Deep Fields (HDFs). Here we report that with ACS WFC data using less than ∼1/10 the exposure of the WFPC2 HDFs, it is possible to confidently carry out resolved studies of faint galaxies at similar magnitude limits. We also investigate the existence of a population of morphologically classified spheroids that show extreme variation in their internal color properties, similar to the ones reported in the HDFs. These are displayed as blue cores and inverse color gradients with respect to those accounted for from metallicity variations. Following the same analysis, we find a similar fraction of early-type systems (∼30%-40%) that show nonhomologous internal colors, suggestive of recent star formation activity. We present two statistics for quantifying the internal color variation in galaxies and tracing blue cores, from which we estimate the ratio of nonhomogeneous to homogeneous internal colors as a function of redshift up to z≲1.2. We find that it can be described as about constant as a function of redshift, with a small increase with redshift for the fraction of spheroids that present strong color dispersions. The implications of a constant fraction at all redshifts suggests the existence of a relatively permanent population of evolving spheroids up to z≲1. We discuss the implications of this in the context of spheroidal formation.