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Published July 2012 | Published
Journal Article Open

The velocity dispersion and mass function of the outer halo globular cluster Palomar 4

Abstract

We obtained precise line-of-sight radial velocities of 23 member stars of the remote halo globular cluster Palomar 4 (Pal 4) using the High Resolution Echelle Spectrograph at the Keck I telescope.We also measured the mass function of the cluster down to a limiting magnitude of V ∼ 28 mag using archival Hubble Space Telescope/Wide Field Planetary Camera 2 (WFPC2) imaging. We derived the cluster's surface brightness profile based on the WFPC2 data and on broad-band imaging with the Low Resolution Imaging Spectrometer at the Keck II telescope. We find a mean cluster velocity of 72.55 ± 0.22 km s^(−1) and a velocity dispersion of 0.87 ± 0.18 km s^(−1). The global mass function of the cluster, in the mass range 0.55 ≤ M ≤ 0.85M_⊙, is shallower than a Kroupa mass function and the cluster is significantly depleted in low-mass stars in its centre compared to its outskirts. Since the relaxation time of Pal 4 is of the order of a Hubble time, this points to primordial mass segregation in this cluster. Extrapolating the measured mass function towards lower mass stars and including the contribution of compact remnants, we derive a total cluster mass of 29 800M_⊙. For this mass, the measured velocity dispersion is consistent with the expectations of Newtonian dynamics and below the prediction of modified Newtonian dynamics (MOND). Pal 4 adds to the growing body of evidence that the dynamics of star clusters in the outer Galactic halo can hardly be explained by MOND.

Additional Information

© 2012 The Authors. Monthly Notices of the Royal Astronomical Society © 2012 RAS. Accepted 2012 April 13. Received 2012 April 13; in original form 2011 November 30. Article first published online: 16 May 2012. We thank Katrin Jordi for providing her SDSS-based surface density profile of Pal 4. We also thank the referee of this paper, Antonio Sollima, for the useful report that helped improve the manuscript. This work was partially supported by Sonderforschungsbereich 881, 'The Milky Way System' (subprojects A2 and A3) of the German Research Foundation (DFG) at the University of Heidelberg. HB acknowledges support from the Australian Research Council through Future Fellowship grant FT0991052. SGD acknowledges a partial support from the NSF grants AST- 0407448 and AST-0909182. Some of the data presented herein were 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. The Observatory was made possible by the generous financial support of the W.M. Keck Foundation. Based in part on observations made with the NASA/ESA Hubble Space Telescope, obtained from the Multimission Archive at the Space Telescope Science Institute (MAST). STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555. This research has made use of the NASA/IPAC Infrared Science Archive, which is operated by the Jet Propulsion Laboratory, California Institute of Technology, under contract with NASA.

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August 22, 2023
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