The Least Luminous Galaxies in the Universe: Clues to the "Missing
Galaxy" problem
Beth Willman
Over the past few years, theoretical and observational studies of
Milky Way dwarf galaxies have flourished. Milky Way dwarfs are close
enough to enable the measurement of their star formation histories and
their spatial and kinematic structures with unprecedented detail. The
'missing galaxy' and the 'cusp/core' problems with CDM cosmologies
have also motivated new interest in these nearby dwarfs. To create a
well-defined dwarf galaxy sample to the faintest possible limits, we
have been conducting a uniform survey of SDSS I for ultra-faint Milky
Way dwarfs. I present the detection limits and current status of this
search. Two new companions have thus far been confirmed as a result
of this search: Willman 1 and the Ursa Major dwarf, the lowest
luminosity galaxy yet known. Although 40 kpc away, Willman 1 displays
very strong evidence for tidal stripping. The properties of these two
objects raise the questions: What is the relationship between globular
clusters and dwarf galaxies? and What is the lower limit of galaxy
formation? I discuss the results of this search in the context of
Cold Dark Matter cosmological models. I also discuss the implications
our search has for the stellar content of the High-Velocity clouds.