Structure and mass of a young globular cluster in NGC 6946

Using the Wide Field Planetary Camera 2 on board the Hubble Space Telescope, we have imaged a luminous young star cluster in the nearby spiral galaxy NGC 6946. Within a radius of 65 pc, the cluster has an absolute visual magnitude, M-V = -13.2, comparable to the most luminous young "super star clusters" in the Antennae merger galaxy. UBV colors indicate an age of about 15 Myr. The cluster has a compact core (radius similar to 1.3 pc) surrounded by an extended envelope with a power-law luminosity profile. The outer parts of the cluster profile gradually merge with the general field, making it difficult to measure a precise half-light radius R-e, but we estimate R-e similar to 13 pc. Combined with population synthesis models, the luminosity and age of the cluster imply a mass of 8.2 x 10(5) M. for a Salpeter initial mass function (IMF) extending down to 0.1 M.. If the IMF is lognormal below 0.4 M., then the mass decreases to 5.5 x 10(5) M.. Depending on model assumptions, the central density of the cluster is between 5.3 X 10(3) and 1.7 x 10(4) M. pc(-3), comparable to other high-density star-forming regions. We also estimate a dynamical mass for the cluster using high-dispersion spectra from the HIRES spectrograph on the Keck I telescope. The HIRES data indicate a velocity dispersion of 10.0 +/- 2.7 km s(-1) and imply a total cluster mass within 65 pe of (1.7 +/- 0.9) x 10(6) M.. Comparing the dynamical mass with the mass estimates based on the photometry and population synthesis models, we find that the mass-to-light ratio is at least as high as for a Salpeter IMF extending down to 0.1 M., although a turnover in the IMF at 0.4 M. is still possible within the similar to 1 sigma errors. The cluster will presumably remain bound, evolving into a globular cluster-like object.