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Title:
Kinematic Masses of Super-Star Clusters in M82 from High-Resolution Near-Infrared Spectroscopy
Authors:
McCrady, Nate; Gilbert, Andrea M.; Graham, James R.
Affiliation:
AA(Department of Astronomy, University of California, Berkeley, CA 94720-3411 ), AB(Department of Astronomy, University of California, Berkeley, CA 94720-3411 ), AC(Department of Astronomy, University of California, Berkeley, CA 94720-3411 )
Publication:
The Astrophysical Journal, Volume 596, Issue 1, pp. 240-252. (ApJ Homepage)
Publication Date:
10/2003
Origin:
UCP
ApJ Keywords:
Galaxies: Individual: Messier Number: M82, Galaxies: Nuclei, Galaxies: Starburst, Galaxies: Star Clusters, Infrared: Galaxies
DOI:
10.1086/377631
Bibliographic Code:
2003ApJ...596..240M

Abstract

Using high-resolution (R~22,000) near-infrared (1.51-1.75 μm) spectra from Keck Observatory, we measure the kinematic masses of two super-star clusters in M82. Cross-correlation of the spectra with template spectra of cool evolved stars gives stellar velocity dispersions of σr=15.9+/-0.8 km s-1 for J0955505+694045 (MGG-9) and σr=11.4+/-0.8 km s-1 for J0955502+694045 (MGG-11). The cluster spectra are dominated by the light of red supergiants and correlate most closely with template supergiants of spectral types M0 and M4.5. King model fits to the observed profiles of the clusters in archival Hubble Space Telescope/Near-Infrared Camera and Multi-Object Spectometer images give half-light radii of rhp=2.6+/-0.4 pc for MGG-9 and rhp=1.2+/-0.17 pc for MGG-11. Applying the virial theorem, we determine masses of 1.5+/-0.3×106 Msolar for MGG-9 and 3.5+/-0.7×105 Msolar for MGG-11 (where the quoted errors include σr, rhp, and the distance). Population synthesis modeling suggests that MGG-9 is consistent with a standard initial mass function (IMF), whereas MGG-11 appears to be deficient in low-mass stars relative to a standard IMF. There is, however, evidence of mass segregation in the clusters, in which case the virial mass estimates would represent lower limits.

Based on observations made at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation.

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