Structural Parameters and Dynamical Masses for Globular Clusters in M33

Abstract

Using high-dispersion spectra from the HIRES echelle spectrograph on the Keck I telescope, we measure velocity dispersions for four globular clusters (GCs) in M33. Combining the velocity dispersions with integrated photometry and structural parameters derived from King-Michie model fits to Wide Field Planetary Camera 2 images, we obtain mass-to-light ratios for the clusters. The mean value is M/L_V=1.53+/-0.18, very similar to the M/L_V of Milky Way and M31 GCs. The M33 clusters also fit very well into the fundamental plane and binding energy-luminosity relations derived for Milky Way GCs. Dynamically and structurally, the four M33 clusters studied here appear virtually identical to Milky Way and M31 GCs.

Based on data obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration, and with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555.