Notes on the Missing Satellites Problem
  The Missing Satellites Problem (MSP) broadly refers to the overabundance of
predicted Cold Dark Matter (CDM) subhalos compared to satellite galaxies known
to exist in the Local Group. The most popular interpretation of the MSP is that
the smallest dark matter halos in the universe are extremely inefficient at
forming stars. The question from that standpoint is to identify the feedback
source that makes small halos dark and to identify any obvious mass scale where
the truncation in the efficiency of galaxy formation occurs. Among the most
exciting developments in near-field cosmology in recent years is the discovery
of a new population satellite galaxies orbiting the Milky Way and M31. Wide
field, resolved star surveys have more than doubled the dwarf satellite count
in less than a decade, revealing a population of ultrafaint galaxies that are
less luminous that some star clusters. For the first time, there are empirical
reasons to believe that there really are missing satellite galaxies in the
Local Group, lurking just beyond our ability to detect them, or simply
inhabiting a region of the sky that has yet to have been surveyed. Both
kinematic studies and completeness-correction studies seem to point to a
characteristic potential well depth for satellite subhalos that is quite close
to the mass scale where photoionization and atomic cooling should limit galaxy
formation. Among the more pressing problems associated with this interpretation
is to understand the selection biases that limit our ability to detect the
lowest mass galaxies. The least massive satellite halos are likely to host
stealth galaxies with very-low surface brightness and this may be an important
limitation in the hunt for low-mass fossils from the epoch of reionization.
