Ultracompact Blue Dwarf Galaxies:Hubble Space TelescopeImaging and Stellar Population Analysis

We present deep Hubble Space Telescope Advanced Camera for Surveys / HighResolution Channel U, narrow-V, and I images of nine "ultracompact" blue dwarfgalaxies (UCBDs) selected from the Sloan Digital Sky Survey. We define UCBDs aslocal (z < 0.01) star-forming galaxies having angular diameters < 6" andphysical sizes < 1 kpc. They are also among the most metal-poor galaxies known,and are found to reside within voids. Both the HST images and the objects'optical spectra reveal that they are composites of young (~1 Myr) populationsthat dominate their light, and older (~10 Gyr) populations that dominate theirstellar masses, which we estimate to be ~10^7 - 10^8 Msol. An intermediate-agepopulation is also indicated in most cases. The objects are not as dynamicallydisturbed as the prototype UCBD, POX 186, but the structure of several of themsuggests that their current starbursts have been triggered by thecollisions/mergers of smaller clumps of stars. In one case, HS 0822+3542, theACS/HRC images resolve the object into two small (~100 pc) components whichappear to have recently collided, supporting this interpretation. In six of theobjects much of their star formation is concentrated in Young Massive Starclusters. The evidence that the galaxies consist mainly of ~10 Gyr old starsestablishes that they are not protogalaxies; their low metallicities are morelikely to be the result of the escape of supernova ejecta, as opposed to youth.These results are consistent with recent galaxy formation simulations whichpredict that cosmic re-ionization at z ~ 6 significantly limited the subsequentstar formation of dwarf galaxies in voids due to the photo-evaporation ofbaryons from their cold dark matter halos (Abridged).Comment: 43 pages, 6 figures, accepted for publication in ApJ; figures available at ftp://ftp.nofs.navy.mil/pub/outgoing/mcorbin