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Dark matter in dwarf spheroidals – II. Observations and modelling of Draco |
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| Authors: | JKleyna M IWilkinson N WEvans GGilmore CFrayn |
| Institution: | Institute of Astronomy, Madingley Rd, Cambridge CB3 0HA; Theoretical Physics, Department of Physics, 1 Keble Road, Oxford OX1 3NP |
| Abstract: | We present stellar radial velocity data for the Draco dwarf spheroidal (dSph) galaxy obtained using the AF2/WYFFOS instrument combination on the William Herschel Telescope. Our data set consists of 186 member stars, 159 of which have good quality velocities, extending to a magnitude V ≈19.5 with a mean velocity precision of ≈2 km s We find statistically strong evidence of a rising velocity dispersion consistent with a dark matter halo that has a gently rising rotation curve. There is a <2 σ signature of rotation about the long axis, inconsistent with tidal disruption as the source of the rising dispersion. By comparing our data set with earlier velocities, we find that Draco probably has a binary distribution and fraction comparable to those in the solar neighbourhood. We apply a novel maximum likelihood algorithm and fit the velocity data to a two parameter spherical model with an adjustable dark matter content and velocity anisotropy. Draco is best fit by a weakly tangentially anisotropic distribution of stellar orbits in a dark matter halo with a very slowly rising rotation law ( v |
| Keywords: | stellar dynamics galaxies: individual: Draco galaxies: kinematics and dynamics Local Group dark matter |