Counting Dark Sub-halos with Star Stream Gaps
  The Cold Dark Matter paradigm predicts vast numbers of dark matter sub-halos
to be orbiting in galactic halos. The sub-halos are detectable through the gaps
they create gaps in stellar streams. The gap-rate is an integral over the
density of sub-halos, their mass function, velocity distribution and the
dynamical age of the stream. The rate of visible gap creation is a function of
the width of the stream. The available data for four streams: the NW stream of
M31, the Pal~5 stream, the Orphan Stream and the Eastern Banded Structure, are
compared to the LCDM predicted relation. We find a remarkably good agreement,
although there remains much to be done to improve the quality of the result.
The narrower streams require that there is a total population of order 10^5
sub-halos above 10^5 M_sun to create the gaps.
