Palomar 13: An Unusual Stellar System in the Galactic Halo

We have measured Keck/HIRES radial velocities for 30 candidate red giants inthe direction of Palomar 13: an object traditionally cataloged as a compact,low-luminosity globular cluster. From a sample of 21 confirmed members, we finda systemic velocity of 24.1 km/s and a projected, intrinsic velocity dispersionof 2.2 km/s. Although small, this dispersion is several times larger than thatexpected for a globular cluster of this luminosity and central concentration.Taken at face value, this dispersion implies a mass-to-light ratio of ~ 40 (insolar units) based on the best-fit King-Michie model. The surface densityprofile of Palomar 13 also appears to be anomalous among Galactic globularclusters -- depending upon the details of background subtraction andmodel-fitting, Palomar 13 either contains a substantial population of"extra-tidal" stars, or it is far more spatially extended than previouslysuspected. The full surface density profile is equally well-fit by aKing-Michie model having a high concentration and large tidal radius, or by aNFW model. We examine -- and tentatively reject -- a number of possibleexplanations for the observed characteristics of Palomar 13 (e.g., velocity"jitter" among the red giants, spectroscopic binary stars, non-standard massfunctions, modified Newtonian dynamics), and conclude that the two mostplausible scenarios are either catastrophic heating during a recentperigalacticon passage, or the presence of a massive dark halo. Thus, theavailable evidence suggests that Palomar 13 is either a globular cluster whichis now in the process of dissolving into the Galactic halo, or a faint,dark-matter-dominated stellar system (ABRIDGED).