Palomar 5 and its tidal tails: A search for new members in the tidal stream

Abstract

In this paper, we present the results of a search for members of the globular cluster Palomar 5 and its associated tidal tails. The analysis has been performed using intermediate and lowresolution spectroscopy with the AAOmega spectrograph on the Anglo-Australian Telescope. Based on kinematics, line strength and photometric information, we identify 39 new red giant branch stars along ~20° of the tails, a larger angular extent than has been previously studied. We also recover eight previously known tidal tail members. Within the cluster, we find seven new red giant and one blue horizontal branch members and confirm a further 12 known red giant members. In total, we provide velocity data for 67 stars in the cluster and the tidal tails. Using a maximum likelihood technique, we derive a radial velocity for Pal 5 of -57.4 ± 0.3 km s-1 and a velocity dispersion of 1.2 ± 0.3 km s-1.We confirm and extend the linear velocity gradient along the tails of 1.0 ± 0.1 km s-1 deg-1, with an associated intrinsic velocity dispersion of 2.1 ± 0.4 km s-1. Neither the velocity gradient nor the dispersion change in any significant way with angular distance from the cluster, although there is some indication that the gradient may be smaller at greater angular distances in the trailing tail. Our results verify the tails as kinematically cold structures and will allow further constraints to be placed on the orbit of Pal 5, ultimately permitting a greater understanding of the shape and extent of the Galaxy's dark matter halo.