Ultracompact blue dwarf galaxies:: Hubble Space Telescope imaging and stellar population analysis

We present deep Hubble Space Telescope (HST) Advanced Camera for Surveys/High Resolution Channel U-, narrow-V-, and I-band images of nine "ultracompact'' blue dwarf galaxies (UCBDs) selected from the Sloan Digital Sky Survey (SDSS). We define UCBDs as local (z < 0.01) star-forming galaxies having angular diameters less than 600 and physical diameters < 1 kpc. They are also among the most metal-poor galaxies known, including objects having 12 + log(O/H) < 7.65, and are found to reside within voids. Both the HST images and the objects' SDSS optical spectra reveal that they are composites of young (similar to 1-10 Myr) populations that dominate their light and older (similar to 10 Gyr) populations that dominate their stellar masses, which we estimate to be similar to 10(7)-10(8) M-circle dot. An intermediate-age (similar to 10(7)-10(9) yr) population is also indicated in most objects. The objects do not appear to be as dynamically disturbed as the prototype UCBD, POX 186, but the structure of several of them suggests that their current star formation has been triggered by the collisions/mergers of smaller clumps of stars. In one case, HS 0822+3542, the images resolve what may be two small (similar to 100 pc) components that have recently collided, supporting this interpretation. In six of the objects much of the star formation is concentrated in young massive clusters, contributing to their compactness in ground-based images. The evidence that the galaxies consist mainly of similar to 10 Gyr old stars establishes that they are not protogalaxies, forming their first generation of stars. Their low metallicities are more likely to be the result of the escape of supernova ejecta, rather than youth.