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 ~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~13 pc. Combined with population synthesis models, the luminosity and age of the cluster imply a mass of 8.2×10⁵ M_solar for a Salpeter initial mass function (IMF) extending down to 0.1 M_solar. If the IMF is lognormal below 0.4 M_solar, then the mass decreases to 5.5×10⁵ M_solar. Depending on model assumptions, the central density of the cluster is between 5.3×10³ and 1.7×10⁴ M_solar 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 pc of (1.7+/-0.9)×10⁶ M_solar. 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_solar, although a turnover in the IMF at 0.4 M_solar is still possible within the ~1 σ errors. The cluster will presumably remain bound, evolving into a globular cluster-like object. Based on observations with the NASA/ESA Hubble Space Telescope and with the W. M. Keck Telescope.