Dynamical Evidence for a Luminous, Young Globular Cluster in NGC 1569

Recent high-resolution observations with the Hubble Space Telescope (HST) reveal that star clusters of extraordinary luminosity and compactness are commonly found in a variety of starburst systems. There has been much speculation that these clusters represent present-day analogs of young globular clusters. Using the HIRES echelle spectrograph on the Keck 10 m telescope, we obtained high-dispersion optical spectra of one of the super star clusters'' (cluster A’') in the nearby dwarf galaxy NGC 1569. The size of the cluster is known from published HST images. The line-of-sight velocity dispersion (15.7±1.5 km/s) has been measured from a cross-correlation analysis of its integrated spectrum at visual wavelengths. If the cluster is gravitationally bound and the velocities are isotropic, application of the virial theorem implies that the cluster has a total stellar mass of (3.3±0.5) X 10${}^5$ solar masses. This object’s mass, mass density, and probable mass-to-light ratio after aging 10–15 Gyr are fully consistent with the typical values of Galactic globular clusters. Our result strongly suggests that at least some of the luminous, compact, young star clusters being discovered with {\it HST} will indeed evolve into normal globular clusters of the type seen in the Milky Way.