  We present deep Hubble Space Telescope Advanced Camera for Surveys / High
Resolution Channel U, narrow-V, and I images of nine "ultracompact" blue dwarf
galaxies (UCBDs) selected from the Sloan Digital Sky Survey. We define UCBDs as
local (z < 0.01) star-forming galaxies having angular diameters < 6" and
physical 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) populations
that dominate their light, and older (~10 Gyr) populations that dominate their
stellar masses, which we estimate to be ~10^7 - 10^8 Msol. An intermediate-age
population is also indicated in most cases. The objects are not as dynamically
disturbed as the prototype UCBD, POX 186, but the structure of several of them
suggests that their current starbursts have been triggered by the
collisions/mergers of smaller clumps of stars. In one case, HS 0822+3542, the
ACS/HRC images resolve the object into two small (~100 pc) components which
appear to have recently collided, supporting this interpretation. In six of the
objects much of their star formation is concentrated in Young Massive Star
clusters. The evidence that the galaxies consist mainly of ~10 Gyr old stars
establishes that they are not protogalaxies; their low metallicities are more
likely to be the result of the escape of supernova ejecta, as opposed to youth.
These results are consistent with recent galaxy formation simulations which
predict that cosmic re-ionization at z ~ 6 significantly limited the subsequent
star formation of dwarf galaxies in voids due to the photo-evaporation of
baryons from their cold dark matter halos (Abridged).
