HST Imaging of the Local Volume Dwarf Galaxies Pisces A&B: Prototypes
  for Local Group Dwarfs
  We present observations of the Pisces A and B galaxies with the Advanced
Camera for Surveys on the Hubble Space Telescope. Photometry from these images
clearly resolve a Red Giant Branch for both objects, demonstrating that they
are nearby dwarf galaxies. We describe a Bayesian inferential approach to
determining the distance to these galaxies using the magnitude of the tip of
the RGB, and then apply this approach to these galaxies. We also provide the
full probability distributions for parameters derived using this approach. This
reveals the distance to these galaxies as $5.64^{+0.13}_{-0.15} \, {\rm Mpc}$
and $8.89^{+0.75}_{-0.85} \, {\rm Mpc}$ for Pisces A and B, respectively,
placing both within the Local Volume but not the Local Group. We estimate the
star formation histories of these galaxies, which suggests that they have
recently undergone an increase in their star formation rates. Together these
yield luminosities for Pisces A and B of $M_V=-11.57^{+0.06}_{-0.05}$ and
$-12.9 \pm 0.2$, respectively, and estimated stellar masses of
$\log(M_*/M_{\odot})= 7.0^{+0.4}_{-1.7}$ and $7.5^{+0.3}_{-1.8}$. We further
show that these galaxies are likely at the boundary between nearby voids and
higher-density filamentary structure. This suggests that they are entering a
higher-density region from voids, where they would have experienced delayed
evolution, consistent with their recent increased star formation rates. If this
is indeed the case, they are useful for study as proxies of the galaxies that
later evolved into typical Local Group satellite galaxies.
