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 \mathrm { km\ > s ^ { -1 } } 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 } \mathrm { km\ > s ^ { -1 } } 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 \mathrm { km\ > s ^ { -1 } } 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 . keywords : Dark Matter : Substructure , Dwarf Galaxies : Ursa Minor , Bayesian Statistics