Kinematic Masses of Super–Star Clusters in M82 from High‐Resolution Near‐Infrared Spectroscopy

Using high-resolution (R~22,000) near-infrared (1.51 -- 1.75 microns) spectrafrom Keck Observatory, we measure the kinematic masses of two super starclusters in M82. Cross-correlation of the spectra with template spectra of coolevolved stars gives stellar velocity dispersions of sigma_r=15.9 +/- 0.8 km/sfor MGG-9 and sigma_r=11.4 +/- 0.8 km/s for MGG-11. The cluster spectra aredominated by the light of red supergiants, and correlate most closely withtemplate supergiants of spectral types M0 and M4.5. We fit King models to theobserved profiles of the clusters in archival HST/NICMOS images to measure thehalf-light radii. Applying the virial theorem, we determine masses of 1.5 +/-0.3 x 10^6 M_sun for MGG-9 and 3.5 +/- 0.7 x 10^5 M_sun for MGG-11. Populationsynthesis modelling suggests that MGG-9 is consistent with a standard initialmass function, whereas MGG-11 appears to be deficient in low-mass starsrelative to a standard IMF. There is, however, evidence of mass segregation inthe clusters, in which case the virial mass estimates would represent lowerlimits.Comment: 16 pages, 8 figures; ApJ, in pres