An Algorithm for Reconstructing the Orphan Stream Progenitor with
  MilkyWay@home Volunteer Computing
  We have developed a method for estimating the properties of the progenitor
dwarf galaxy from the tidal stream of stars that were ripped from it as it fell
into the Milky Way. In particular, we show that the mass and radial profile of
a progenitor dwarf galaxy evolved along the orbit of the Orphan Stream,
including the stellar and dark matter components, can be reconstructed from the
distribution of stars in the tidal stream it produced. We use MilkyWay@home, a
PetaFLOPS-scale distributed supercomputer, to optimize our dwarf galaxy
parameters until we arrive at best-fit parameters. The algorithm fits the dark
matter mass, dark matter radius, stellar mass, radial profile of stars, and
orbital time. The parameters are recovered even though the dark matter
component extends well past the half light radius of the dwarf galaxy
progenitor, proving that we are able to extract information about the dark
matter halos of dwarf galaxies from the tidal debris. Our simulations assumed
that the Milky Way potential, dwarf galaxy orbit, and the form of the density
model for the dwarf galaxy were known exactly; more work is required to
evaluate the sources of systematic error in fitting real data. This method can
be used to estimate the dark matter content in dwarf galaxies without the
assumption of virial equilibrium that is required to estimate the mass using
line-of-sight velocities. This demonstration is a first step towards building
an infrastructure that will fit the Milky Way potential using multiple tidal
streams.
