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S. S. Larsen, J. P. Brodie (UCO / Lick Observatory, UC Santa Cruz), Y. N. Efremov (Sternberg Astronomical Institute, Moscow, Russia), B. G. Elmegreen (IBM Research Division, T. J. Watson Research Center), P. W. Hodge (University of Washington Astronomy Department), T. Richtler (Grupo de Astronomi, Universidad de Concepcion, Chile)
Using new HST / WFPC2 data, we have carried out a study of a peculiar star forming region in the nearby Sc-type spiral galaxy NGC 6946. The region has a bubble-like shape and a diameter of about 600 pc. Within the bubble we find ~ 20 young star clusters with MV < -8 and near the center is an extremely luminous young globular cluster with MV=-13. The UBV colors of the clusters are consistent with all of them having similar ages of about 15 Myr but somewhat different reddenings. The radial profile of the young globular is well resolved on our PC images and we estimate its effective radius to be about 6 pc.
We have also obtained Keck / HIRES spectra of the young globular. From these data we estimate a velocity dispersion of about 11 km / sec, leading to a dynamical mass estimate of 2±1 \times 106 solar masses. This is in good agreement with the mass predicted by Bruzual & Charlot population synthesis models and the assumption of a Salpeter or Scalo stellar mass function in the range 0.1-100 solar masses. Thus, the IMF of this cluster appears to be normal, without any evidence for lack of low-mass stars. It is therefore likely that the young globular will eventually evolve into an object quite similar to the globular clusters in the halo of the Milky Way.
Support for this work was provided by NASA through a grant from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555, and by NSF grant number AST9900732.