Palomar 13: An Unusual Stellar System in the Galactic Halo (original) (raw)

2002, The Astrophysical Journal

We report the first results of a program to study the internal kinematics of globular clusters in the outer halo of the Milky Way. Using the Keck telescope and High Resolution Echelle Spectrometer, we have measured precise radial velocities for 30 candidate red giants in the direction of Palomar 13, an object traditionally cataloged as a compact, low-luminosity globular cluster. We have combined these radial velocities with published proper motion membership probabilities and new CCD photometry from the Keck and Canada-France-Hawaii telescopes, to isolate a sample of 21 probable members. We find a systemic velocity of v r s = 24.1±0.5 km s −1 and a projected, intrinsic velocity dispersion of σ p = 2.2±0.4 km s −1 . Although small, this dispersion is nevertheless several times larger than that expected for a globular cluster of this luminosity and central concentration. Taken at face value, this dispersion implies a mass-to-light ratio of Υ V = 40 +24 −17 based on the best-fit King-Michie model. The surface density profile of Palomar 13 also appears to be anomalous among Galactic globular clusters -depending upon the details of background subtraction and model-fitting, Palomar 13 either contains a substantial population of "extra-tidal" stars, or it is far more spatially extended than previously suspected. The full surface density profile is equally well-fit by a King-Michie model having a high concentration and large tidal radius, or by a NFW model. We examine -and tentatively reject -a number of possible explanations for the observed characteristics of Palomar 13 (e.g., velocity "jitter" among the red giants, spectroscopic binary stars, non-standard mass functions, modified Newtonian dynamics), and conclude that the two most plausible scenarios are either catastrophic heating during a recent perigalacticon passage, or the presence of a massive dark halo. Thus, the available evidence suggests that Palomar 13 is either a globular cluster which is now in the process of dissolving into the Galactic halo, or a faint, dark-matter-dominated stellar system.