Hubble Space Telescope Observations of NGC 253 Dwarf Satellites:
  Discovery of Three Ultra-faint Dwarf Galaxies
  We present deep Hubble Space Telescope imaging of five faint dwarf galaxies
associated with the nearby spiral NGC 253 (D$\approx$3.5 Mpc). Three of these
are newly discovered ultra-faint dwarf galaxies, while all five were found in
the Panoramic Imaging Survey of Centaurus and Sculptor (PISCeS), a
Magellan$+$Megacam survey to identify faint dwarfs and other substructures in
resolved stellar light around massive galaxies outside of the Local Group. Our
HST data reach $\gtrsim$3 magnitudes below the tip of the red giant branch for
each dwarf, allowing us to derive their distances, structural parameters, and
luminosities. All five systems contain predominantly old, metal-poor stellar
populations (age$\sim$12 Gyr, [M/H]$\lesssim$$-$1.5) and have sizes
($r_{h}$$\sim$110-3000 pc) and luminosities ($M_V$$\sim$$-7$ to $-12$ mag)
largely consistent with Local Group dwarfs. The three new NGC 253 satellites
are among the faintest systems discovered beyond the Local Group. We also use
archival HI data to place limits on the gas content of our discoveries. Deep
imaging surveys such as our program around NGC 253 promise to elucidate the
faint end of the satellite luminosity function and its scatter across a range
of galaxy masses, morphologies, and environments in the decade to come.
