Observational evidence for the present-day formation of globular clusters

We present evidence that some of the compact, luminous, young star clusters recently discovered through images taken with the Hubble Space Telescope (HST) have masses comparable to those of old Galactic globular clusters. Three “super star clusters” in the centers of the nearby dwarf galaxies NGC 1569, NGC 1705, and NGC 4214 have been observed with the HIRES echelle spectrograph on the Keck 10 m telescope to measure the velocity dispersion of the stars. The velocity dispersion was measured successfully for the clusters in NGC 1569 and NGC 1705, which, when combined with the size estimates obtained from HST images, imply that the clusters have very large dynamical masses. The masses, mass densities, and predicted mass-to-light ratios (at t=10–15 Gyr) of these two 10-Myr old clusters closely resemble those of the majority of evolved Galactic globular clusters. We interpret the results as evidence that these objects are genuinely young globular clusters.