The Missing Satellite Problem Outside of the Local Group: I -- Pilot
  Observation
  We present results from a pilot observation of nearby (~20 Mpc) galaxies with
mass similar to that of the Milky Way (MW) to address the missing satellite
problem. This is the first paper from an on-going project to address the
problem with a statistical sample of galaxies outside of the Local Group (LG)
without employing an assumption that the LG is a typical halo in the Universe.
Thanks to the close distances of our targets, dwarf galaxies around them can be
identified as extended, diffuse galaxies. By applying a surface brightness cut
together with a careful visual screening to remove artifacts and background
contamination, we construct a sample of dwarf galaxies. The luminosity function
(LF) of one of the targets is broadly consistent with that of the MW, but the
other has a more abundant dwarf population. Numerical simulations by Okamoto
(2013) seem to overpredict the number of dwarfs on average, while more recent
predictions from Copernicus Complexio are in a better agreement. In both
observations and simulations, there is a large diversity in the LFs,
demonstrating the importance of addressing the missing satellite problem with a
statistically representative sample. We also characterize the projected spatial
distributions of the satellites and do not observe strong evidence for
alignments around the central galaxies. Based on this successful pilot
observation, we are carrying out further observations to increase the sample of
nearby galaxies, which we plan to report in our future paper.
