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.