Kinematics of Star Clusters in M33: Distinct Populations

We present velocity measurements for 107 star clusters in the nearby, low-luminosity spiral galaxy M33, from a homogeneous data set taken with the WIYN HYDRA spectrograph. The target clusters were chosen primarily from our previous catalogs based on multiband Hubble Space Telescope WFPC2 imaging and therefore have previous age estimates based on integrated colors. Our sample includes objects spanning the entire range of cluster ages found in M33, allowing us to study cluster kinematics as a function of age. We find that the cluster velocity dispersion with respect to the local disk motion increases dramatically with age. The young clusters have a maximum circular velocity of 87 ± 11 km s^-1, similar to the previous rotation solution for H II regions in M33. When clusters are divided into age groups, the old clusters clearly have very different kinematics from the young population and from the gas disk. Monte Carlo simulations comparing synthetic disk plus halo systems with the observed velocity distribution for our old M33 clusters suggest that old clusters are likely to be composed of two distinct subpopulations, an 85% ± 5% halo component plus a 15% ± 5% disk component. This is the first evidence for distinct kinematic subpopulations among the old M33 clusters, and this galaxy appears to have a higher halo/bulge fraction of old stellar objects than found in the Galaxy. To further study the properties of halo clusters in M33, we use spectroscopic line index measurements for 17 clusters with the largest departure from disk motion, and thus the most pristine halo objects in our sample, to determine ages and approximate metal abundances. When compared with the Worthey models and with similar measurements for 12 Galactic globulars, our clusters have an age spread of ~5-7 Gyr, ~2 times as large as the spread among the Galactic sample. This small halo sample shows little evidence for a progression in metal abundance with age. There is also some support for different size (core radius) distributions when comparing "young" and "old" halo M33 clusters. We discuss these findings in the context of different galaxy formation models.