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_☉ 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 × 10⁵ M_☉. Depending on model assumptions, the central density of the cluster is between 5.3 × 10³ and 1.7 × 10⁴ 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 pc of × 10⁶ 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 ~1 σ errors. The cluster will presumably remain bound, evolving into a globular cluster-like object.