The Dragonfly Nearby Galaxies Survey. V. HST/ACS Observations of 23 Low
  Surface Brightness Objects in the Fields of NGC1052, NGC1084, M96, and
  NGC4258
  We present HST/ACS imaging of twenty-three very low surface brightness
($\mu_{e,V}$=25-27.5) galaxies detected in the fields of four nearby galaxy
groups. These objects were selected from deep optical imaging obtained with the
Dragonfly Telephoto Array. Seven are new discoveries, while most of the others
had been identified previously in visual surveys of deep photographic plates
and more recent surveys. Few have previously been studied in detail. From the
ACS images, we measure distances to the galaxies using both the tip of the red
giant branch method and the surface brightness fluctuations method. We
demonstrate that the two methods are consistent with each other in the regime
where both can be applied. The distances to 15 out of 20 galaxies with stable
measurements are consistent with that of the targeted group within errors. This
suggests that assuming group membership based solely on projected proximity is
~75% successful in this regime. The galaxies are nearly round, with a median
axis ratio of 0.85, and visually resemble dwarf spheroidal galaxies. The
objects have a range of sizes, from $R_e$=0.4 kpc to $R_e$=1.8 kpc, with a
median of 1.0 kpc. They range in luminosity from $M_V$=-11.4 to $M_V$=-15.6,
with a median of -12.4. Galaxies with $R_e$~1 kpc and $M_V$~-12 are fairly rare
in the Local Group but we find many of them in this relatively small sample.
Four of the objects fall in the class of ultra diffuse galaxies (UDGs), with
$R_e$>1.5 kpc and $\mu_{0,V}$>24 mag/arcsec^2, including the recently
identified dark matter deficient galaxy NGC1052-DF2.
