Pisces VII: Discovery of a possible satellite of Messier 33 in the DESI
  Legacy Imaging Surveys
  We report deep imaging observations with DOLoRes@TNG of an ultra-faint dwarf
satellite candidate of the Triangulum galaxy (M33) found by visual inspection
of the public imaging data release of the DESI Legacy Imaging Surveys. Pisces
VII/Triangulum (Tri) III is found at a projected distance of 72 kpc from M33,
and using the tip of the red giant branch method we estimate a distance of
D=1.0 +0.3,-0.2 Mpc, meaning the galaxy could either be an isolated ultra-faint
or the second known satellite of M33. We estimate an absolute magnitude of
M_V=-6.1+/-0.2 if Pisces VII/Tri II is at the distance of M33, or as bright as
M_V=-6.8+/-0.2 if the galaxy is isolated. At the isolated distance, it has a
physical half-light radius of r_h=131+/-61 pc consistent with similarly faint
galaxies around the Milky Way. As the tip of the red giant branch is sparsely
populated, constraining a precision distance is not possible, but if Pisces
VII/Tri III can be confirmed as a true satellite of M33 it is a significant
finding. With only one potential satellite detected around M33 previously
(Andromeda XXII/Tri I), it lacks a significant satellite population in stark
contrast to the similarly massive Large Magellanic Cloud. The detection of more
satellites in the outskirts of M33 could help to better illuminate if this
discrepancy between expectation and observations is due to a poor understanding
of the galaxy formation process, or if it is due to the low luminosity and
surface brightness of the M33 satellite population which has thus far fallen
below the detection limits of previous surveys. If it is truly isolated, it
would be the faintest known field dwarf detected to date.
