Abstract
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±0.75 \kms and centered at (9.1\arcmin±1.5,7.2\arcmin±1.2) in relative RA and DEC with respect to the center of Ursa Minor. The second object has a large velocity offset of $-12.8^{+1.75}{-1.5}\ \kmscomparedtoUrsaMinorandcenteredat(-14.0\arcmin^{+2.4}{-5.8}, -2.5\arcmin^{+0.4}_{-1.0}).Thekinematicallycoldobjecthasbeenfoundbeforeusingasmallerdatasetbutthepredictionthatthiscoldobjecthasavelocitydispersionlargerthan2.0\ \kms$ 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.
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URL
https://arxiv.org/abs/1208.4146