  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 Dark Energy Camera Legacy Survey.
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 find a distance to
this faint system of D=820^(+195, -190) kpc. We estimate an absolute magnitude
of M_V=-4.4^(+0.8,-0.7) and a half-light radius of r_half =100 +/-14 pc for the
galaxy, 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 difficult, but if Pisces VII/Tri III can be confirmed as a true
satellite of M33, it is a significant finding. Firstly, it would be the
faintest dwarf galaxy detected to-date outside of the Milky Way. 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.
