Hubble Space Telescope Imaging of the Ultracompact Blue Dwarf Galaxy HS 0822+3542: An Assembling Galaxy in a Local Void?

We present deep U, narrow-V, and I-band images of the ultracompact blue dwarfgalaxy HS 0822+3542, obtained with the Advanced Camera for Surveys / HighResolution Channel of the Hubble Space Telescope. This object is extremelymetal-poor (12 + log(O/H) = 7.45) and resides in a nearby void. The imagesresolve it into two physically separate components that were previouslydescribed as star clusters in a single galaxy. The primary component is only\~100 pc in maximum extent, and consists of starburst region surrounded by aring-like structure of relatively redder stars. The secondary component is ~50pc in size and lies at a projected distance of ~80 pc away from the primary,and is also actively star-forming. We estimate masses ~10^7 M(sol) and ~10^6M(sol) for the two components, based on their luminosities, with an associateddynamical timescale for the system of a few Myr. This timescale and thestructure of the components suggests that a collision between them triggeredtheir starbursts. The spectral energy distributions of both components can befitted by the combination of recent (few Myr old) starburst and an evolved(several Gyr old) underlying stellar population, similar to larger blue compactdwarf galaxies. This indicates that despite its metal deficiency the object isnot forming its first generation of stars. However, the small sizes and massesof the two components suggests that HS 0822+3542 represents a dwarf galaxy inthe process of assembling from clumps of stars intermediate in size betweenglobular clusters and objects previously classified as galaxies. Its relativelyhigh ratio of neutral gas mass to stellar mass (~1) and high specific starformation rate, log(SFR/M(sol) = -9.2, suggests that it is still convertingmuch of its gas to stars.Comment: 11 pages, 2 figures, accepted for publication in Astrophysical Journal Letter