Modeling GD-1 Gaps in a Milky-Way Potential
  The GD-1 star stream is currently the best available for identifying density
fluctuations, "gaps", along its length as a test of the LCDM prediction of
large numbers of dark matter sub-halos orbiting in the halo. Density variations
of some form are present, since the variance of the density along the stream is
three times that expected from the empirically estimated variation in the
filtered mean star counts. The density variations are characterized with
filters that approximate the shape of sub-halo gravitationally induced stream
gaps. The filters locate gaps and measures their amplitude, leading to a
measurement of the distribution of gap widths. To gain understanding of the
factors influencing the gap width distribution, a suite of collisionless n-body
simulations for a GD-1 like orbit in a Milky Way-like potential provides a
dynamically realistic statistical prediction of the gap distribution. The
simulations show that every location in the stream has been disturbed to some
degree by a sub-halo. The small gaps found via the filtering are largely noise.
Larger gaps, those longer than 1 kpc, or 10 degrees for GD-1, are the source of
the excess variance. The suite of stream simulations shows that sub-halos at
the predicted inner halo abundance or possibly somewhat higher can produce the
required large sale density variations.
