Evidence for Substructure in Ursa Minor Dwarf Spheroidal Galaxy using a
  Bayesian Object Detection Method
  We present a method for identifying localized secondary populations in
stellar velocity data using Bayesian statistical techniques. We apply this
method to the dwarf spheroidal galaxy Ursa Minor and find two secondary objects
in this satellite of the Milky Way. One object is kinematically cold with a
velocity dispersion of $4.25 \pm 0.75\ \kms$ and centered at $(9.1\arcmin \pm
1.5, 7.2\arcmin \pm 1.2)$ in relative RA and DEC with respect to the center of
Ursa Minor. The second object has a large velocity offset of
$-12.8^{+1.75}_{-1.5}\ \kms$ compared to Ursa Minor and centered at
$(-14.0\arcmin^{+2.4}_{-5.8}, -2.5\arcmin^{+0.4}_{-1.0})$. The kinematically
cold object has been found before using a smaller data set but the prediction
that this cold object has a velocity dispersion larger than $2.0\ \kms$ at 95%
C.L. differs from previous work. We use two and three component models along
with the information criteria and Bayesian evidence model selection methods to
argue that Ursa Minor has one or two localized secondary populations. The
significant probability for a large velocity dispersion in each secondary
object raises the intriguing possibility that each has its own dark matter
halo, that is, it is a satellite of a satellite of the Milky Way.
