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

• Published in: arXiv.org
• Main Authors: Frank, Matthias J, Hilker, Michael, Baumgardt, Holger, Côté, Patrick, Grebel, Eva K, Haghi, Hosein, Küpper, Andreas H W, Djorgovski, S G
• Format: Paper Journal Article
• Language: English
• Published: Ithaca Cornell University Library, arXiv.org 30.11.2016
• Subjects: Dispersion; Image resolution; Low mass stars; Physics - Astrophysics of Galaxies; Physics - Cosmology and Nongalactic Astrophysics; Relaxation time; Star & galaxy formation; Star clusters; Stellar evolution; Surface brightness; Velocity
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.1205.2693

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 (HIRES) 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 HST/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 (LRIS) at the Keck II telescope. We find a mean cluster velocity of 72.55+/-0.22 km/s and a velocity dispersion of 0.87+/-0.18 km/s. The global mass function of the cluster, in the mass range 0.55<=M<=0.85 M_solar, is shallower than a Kroupa mass function and the cluster is significantly depleted in low-mass stars in its center 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 29800 M_solar. 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.