Hubble space telescope imaging of the ultracompact blue dwarf galaxy HS 0822 + 3542: An assembling galaxy in a local void?

We present deep U-band, narrow V-band, and I-band images and photometry of the ultracompact blue dwarf galaxy HS 0822 + 3542 obtained with the Advanced Camera for Surveys/High Resolution Channel of the Hubble Space Telescope. This object is also extremely metal-poor [12 + log (O/H) = 7.45] and resides in a nearby void. The images resolve it into two physically separate components that were previously described as star clusters in a single galaxy. The primary component is only ∼100 pc in maximum extent and consists of a starburst region surrounded by a ringlike structure of relatively redder stars. The secondary component is ∼50 pc in size and lies at a projected distance of ∼80 pc away from the primary, and is also actively star-forming. We estimate masses of ∼10⁷ and ∼10⁶ M _⊙ for the two components based on their luminosities, with an associated dynamical timescale for the system of a few megayears. This timescale and the structure of the components suggest that a collision between them triggered their starbursts. The spectral energy distributions of both components can be fitted by the combination of a recent (a few megayears old) starburst and an evolved (several gigayears old) underlying stellar population, similar to larger blue compact dwarf galaxies. This indicates that despite its metal deficiency, the object is not forming its first generation of stars. However, the small sizes and masses of the two components suggest that HS 0822 + 3542 represents a dwarf galaxy in the process of assembling from clumps of stars intermediate in size between globular clusters and objects previously classified as galaxies. Its relatively high ratio of neutral gas mass to stellar mass (∼1) and high specific star formation rate, log (SFR/M _⊙) ≃ -9.2, also suggest that it is still converting much of its gas to stars.