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Palomar 13: An Unusual Stellar System in the Galactic Halo

Côté, Patrick and Djorgovski, S. G. and Meylan, G. and Castro, Sandra and McCarthy, J. K. (2002) Palomar 13: An Unusual Stellar System in the Galactic Halo. Astrophysical Journal, 574 (2). pp. 783-804. ISSN 0004-637X. doi:10.1086/340978.

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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 modest, this dispersion is nevertheless several times larger than that expected for a globular cluster of this luminosity and central concentration. Taken at face value, it 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 unusual compared to most Galactic globular clusters; depending upon the details of background subtraction and model-fitting, Palomar 13 either contains a substantial population of "extratidal" stars, or is considerably more spatially extended than previously suspected. The full surface density profile is equally well fitted by a King-Michie model having a high concentration and large tidal radius, or by a Navarro-Frenk-White model. We examine—and tentatively reject—a number of possible origins for the observed characteristics of Palomar 13 (e.g., velocity "jitter" among the red giant branch stars, spectroscopic binary stars, nonstandard mass functions, modified Newtonian dynamics) and conclude that the two leading explanations are either catastrophic heating during a recent perigalacticon passage or the presence of a dark matter halo. The available evidence therefore suggests that Palomar 13 is either a globular cluster that is now in the process of dissolving into the Galactic halo or a faint, dark matter-dominated stellar system.

Item Type:Article
Related URLs:
URLURL TypeDescription Paper
Côté, Patrick0000-0003-1184-8114
Djorgovski, S. G.0000-0002-0603-3087
Additional Information:© 2002 The American Astronomical Society. Received 2001 November 9; accepted 2002 April 1. Based on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and NASA, and was made possible by the generous financial support of the W. M. Keck Foundation. We thank Tad Pryor and an anonymous referee for numerous helpful comments. We also thank Gilles Bergond for his assistance with the CFHT observations, and Dean McLaughlin for providing some of the software used in the analysis. P. C. gratefully acknowledges support provided by the Sherman M. Fairchild Foundation during the initial stages of this work. S. J. G. acknowledges partial support from the Bressler Foundation. This work was based in part on observations obtained at the W. M. Keck Observatory, which is operated jointly by the California Institute of Technology and the University of California. We are grateful to the W. M. Keck Foundation for their vision and generosity.
Funding AgencyGrant Number
W. M. Keck FoundationUNSPECIFIED
Sherman Fairchild FoundationUNSPECIFIED
Bressler FoundationUNSPECIFIED
Subject Keywords:Galaxy: halo — Galaxy: kinematics and dynamics — galaxies: dwarf — globular clusters: individual (Palomar 13)
Issue or Number:2
Record Number:CaltechAUTHORS:20190828-103451435
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Official Citation:Patrick Côté et al 2002 ApJ 574 783
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:98313
Deposited By: Tony Diaz
Deposited On:28 Aug 2019 17:45
Last Modified:16 Nov 2021 17:38

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