Deidre A. Hunter, Angela Adamo, et al.
Astronomical Journal
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.
Deidre A. Hunter, Angela Adamo, et al.
Astronomical Journal
Natalia Lahén, Thorsten Naab, et al.
ApJ
Bruce G. Elmegreen, Toshiya Kimura, et al.
Astrophysical Journal
Bruce G. Elmegreen
Astrophysics and Space Science