Radial Velocity and Metallicity of the Globular Cluster IC4499 Obtained with AAOmega

Abstract

We present radial velocity and metallicity measurements for the far-southern Galactic globular cluster IC4499. We selected several hundred target red giant stars in and around the cluster from the 2MASS point source catalog, and obtained spectra at the near-infrared calcium triplet using the AAOmega spectrograph. Observations of giants in globular clusters M4, M22, and M68 were taken to provide radial velocity and metallicity comparison objects. Based on velocity data we conclude that 43 of our targets are cluster members, by far the largest sample of IC4499 giants spectroscopically studied. We determine the mean heliocentric radial velocity of the cluster to be 31.5 plus or minus 0.4 km/s, and find the most likely central velocity dispersion to be 2.5 plus or minus 0.5 km/s. This leads to a dynamical mass estimate for the cluster of 93 plus or minus 37 thousand solar masses. We are sensitive to cluster rotation down to an amplitude of about 1 km/s, but no evidence for cluster rotation is seen. The cluster metallicity is found to be [Fe/H] = -1.52 plus or minus 0.12 on the Carretta-Gratton scale. The radial velocity of the cluster, previously highly uncertain, is consistent with membership in the Monoceros tidal stream, but also with a halo origin. The horizontal branch morphology of the cluster is slightly redder than average for its metallicity, but it is likely not unusually young compared to other clusters of the halo. The new constraints on the cluster kinematics and metallicity may give insight into its extremely high specific frequency of RR Lyrae stars.

Accepted to MNRAS, 13 pages, 9 figures

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