Color, Structure, and Star Formation History of Dwarf Galaxies over the Last ~3 Gyr with GEMS and SDSS

We present a study of the colors, structural properties, and star formation histories for a sample of ~1600 dwarfs over look-back times of ~3 Gyr (z = 0.002-0.25). The sample consists of 401 distant dwarfs drawn from the Galaxy Evolution from Morphologies and SEDs (GEMS) survey, which provides high-resolution Hubble Space Telescope (HST) Advanced Camera for Surveys (ACS) images and accurate redshifts, and also 1291 dwarfs at 10-90 Mpc compiled from the Sloan Digital Sky Survey (SDSS). The sample is complete down to an effective surface brightness of 22 mag arcsec-2 in z and includes dwarfs with Mg = -18.5 to -14 mag. Rest-frame luminosities in Johnson UBV and SDSS ugr filters are provided by the COMBO-17 survey, and structural parameters have been determined by Sérsic fits. We find that the GEMS dwarfs are bluer than the SDSS dwarfs by ~0.13 mag in g - r, which is consistent with the color evolution over ~2 Gyr of star formation histories involving moderate starbursts and long periods of continuous star formation. The full color range of the samples cannot be reproduced by single starbursts of different masses or long periods of continuous star formation alone. Furthermore, an estimate of the mechanical luminosities needed for the gas in the GEMS dwarfs to be completely removed from the galaxies shows that a significant number of low-luminosity dwarfs are susceptible to such a complete gas loss, if they would experience a starburst. On the other hand, a large fraction of more luminous dwarfs is likely to retain their gas. We also estimate the star formation rates per unit area for the GEMS dwarfs and find good agreement with the values for local dwarfs.