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

We obtained precise line-of-sight radial velocities of 23 member stars of the remote halo globular cluster Palomar4 (Pal4) 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~ 28mag 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 \ub1 0.22kms -1 and a velocity dispersion of 0.87 \ub1 0.18kms -1. The global mass function of the cluster, the mass range 0.55 64M 64 0.85M 99, is shallower than a Kroupa mass function and the cluster is significantly depleted low-mass stars its centre compared to its outskirts. Since the relaxation time of Pal4 is of the order of a Hubble time, this points to primordial mass segregation 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 29800M 99. For this mass, the measured velocity dispersion is consistent with the expectations of Newtonian dynamics and below the prediction of modified Newtonian dynamics (MOND). Pal4 adds to the growing body of evidence that the dynamics of star clusters the outer Galactic halo can hardly be explained by MOND. \ua9 2012 The Authors Monthly Notices of the Royal Astronomical Society \ua9 2012 RAS.Peer reviewed: YesNRC publication: Ye