Structure and mass of a young globular cluster in NGC 6946

Abstract

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, MV = -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 Re, but we estimate Re ∼13 pc. Combined with population synthesis models, the luminosity and age of the cluster imply a mass of 8.2 x 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 x 105 M⊙. Depending on model assumptions, the central density of the cluster is between 5.3 x 103 and 1.7 x 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 (1.7 ± 0.9) x 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.