The Outer Halo of the Milky Way as Probed by RR Lyr Variables from the
  Palomar Transient Facility
  RR Lyr stars are ideal massless tracers that can be used to study the total
mass and dark matter content of the outer halo of the Milky Way. This is
because they are easy to find in the light curve databases of large stellar
surveys and their distances can be determined with only knowledge of the light
curve. We present here a sample of 112 RR Lyr beyond 50 kpc in the outer halo
of the Milky Way, excluding the Sgr streams, for which we have obtained
moderate resolution spectra with Deimos on the Keck 2 Telescope. Four of these
have distances exceeding 100 kpc. These were selected from a much larger set of
447 candidate RR Lyr which were datamined using machine learning techniques
applied to the light curves of variable stars in the Palomar Transient Facility
database. The observed radial velocities taken at the phase of the variable
corresponding to the time of observation were converted to systemic radial
velocities in the Galactic standard of rest. From our sample of 112 RR Lyr we
determine the radial velocity dispersion in the outer halo of the Milky Way to
be ~90 km/s at 50 kpc falling to about 65 km/s near 100 kpc once a small number
of major outliers are removed. With reasonable estimates of the completeness of
our sample of 447 candidates and assuming a spherical halo, we find that the
stellar density in the outer halo declines as the -4 power of r.
