* Astronomy

Title: Evidence for Substructure in Ursa Minor Dwarf Spheroidal Galaxy using a Bayesian Object Detection Method
Authors: Andrew B. Pace (1), Gregory D. Martinez (1,2), Manoj Kaplinghat (1), Ricardo R. Muñoz (3, 4), ((1) Center for Cosmology, Department of Physics and Astronomy, University of California, Irvine, (2) The Oskar Klein Center, Department of Physics, Stockholm University, (3) Departamento de Astronomía, Universidad de Chile, (4) Department of Astronomy, Yale University)

We present a method for identifying localised 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 centred at (9.1' ±1.5, 7.2' ±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 centred at (-14.0'^{+2.4}_{-5.8}, -2.5'^{+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 localised 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.

Read more (150kb, PDF)