2001/04/07 by S. S. Larsen, Jean P. Brodie, J. P. Brodie +6 · 3 citations
Physics and Astronomy · #Advanced Camera for Surveys #Astronomy #Astronomy and Astrophysical Research #Astrophysics #Cluster (spacecraft) #Galaxies: Formation, Evolution, Phenomena #Galaxy #Globular cluster #Luminosity #Mass segregation #Physics #RADIUS #Spiral galaxy #Star cluster #Star formation #Stellar mass #Stellar, planetary, and galactic studies #Velocity dispersion #astro-ph
paper · pdf · doi:10.1086/321620
33 pages, including 10 figures and 3 tables. Accepted for publication in the Astrophysical Journal
arxiv created 2001/04/07 · openalex publication_date 2001/08/01 · arxiv updated 2009/12/01 · openalex created_date 2025/10/10 · openalex updated_date 2026/08/05
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 × 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 × 10 5 M ☉ . Depending on model assumptions, the central density of the cluster is between 5.3 × 10 3 and 1.7 × 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 pc of × 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 ~1 σ errors. The cluster will presumably remain bound, evolving into a globular cluster-like object.