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For refcode 2001ApJ...556..801L:
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2001ApJ...556..801L Structure and Mass of a Young Globular Cluster in NGC 6946 Soren S. Larsen and Jean P. Brodie University of California Observatories, Lick Observatory, University of California, Santa Cruz, Santa Cruz, CA 95064; soeren@ucolick.org Bruce G. Elmegreen IBM Research Division, T. J. Watson Research Center, P.O. Box 218, Yorktown Heights, NY 10598 Yuri N. Efremov Sternberg Astronomical Institute, Moscow State University, Moscow 119899, Russia Paul W. Hodge Astronomy Department, University of Washington, Box 351580, Seattle, WA 98195-1580 and Tom Richtler Grupo de Astronomia, Departamento de Fisica, Casilla 160-C, Universidad de Concepcion, Concepcion, Chile Received 2001 February 6; accepted 2001 April 6 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, 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 x 10^5^ M_sun_ for a Salpeter initial mass function (IMF) extending down to 0.1 M_sun_. If the IMF is lognormal below 0.4 M_sun_, then the mass decreases to 5.5 x 10^5^ M_sun_. Depending on model assumptions, the central density of the cluster is between 5.3 x 10^3^ and 1.7 x 10^4^ M_sun_ 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 10^6^ M_sun_. 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_sun_, although a turnover in the IMF at 0.4 M_sun_ is still possible within the ~1 {sigma} errors. The cluster will presumably remain bound, evolving into a globular cluster-like object. Subject headings: galaxies: individual (NGC 6946) - galaxies: star clusters
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