Hubble Space Telescope Imaging of Isolated Local Volume Dwarfs GALFA-Dw3
  and Dw4
  We present observations of the dwarf galaxies GALFA Dw3 and GALFA Dw4 with
the Advanced Camera for Surveys (ACS) on the Hubble Space Telescope (HST).
These galaxies were initially discovered as optical counterparts to compact HI
clouds in the GALFA survey. Both objects resolve into stellar populations which
display an old red giant branch, younger helium burning, and massive main
sequence stars. We use the tip of the red giant branch method to determine the
distance to each galaxy, finding distances of 7.61$_{-0.29}^{+0.28}$ Mpc and
3.10$_{-0.17}^{+0.16}$ Mpc, respectively. With these distances we show that
both galaxies are extremely isolated, with no other confirmed objects within
~1.5 Mpc of either dwarf. GALFA Dw4 is also found to be unusually compact for a
galaxy of its luminosity. GALFA Dw3 and Dw4 contain HII regions with young star
clusters and an overall irregular morphology; they show evidence of ongoing
star formation through both ultraviolet and H$\alpha$ observations and are
therefore classified as dwarf irregulars (dIrrs). The star formation histories
of these two dwarfs show distinct differences: Dw3 shows signs of a recently
ceased episode of active star formation across the entire dwarf, while Dw4
shows some evidence for current star formation in spatially limited HII
regions. Compact HI sources offer a promising method for identifying isolated
field dwarfs in the Local Volume, including GALFA Dw3 & Dw4, with the potential
to shed light on the driving mechanisms of dwarf galaxy formation and
evolution.
