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.