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 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 σ _r = 15.9 ± 0.8 km s^-1 for J0955505+694045 (MGG-9) and σ_r = 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 r_hp = 2.6 ± 0.4 pc for MGG-9 and r_hp = 1.2 ± 0.17 pc for MGG-11. Applying the virial theorem, we determine masses of 1.5 ± 0.3 × 10⁶ M_⊙ for MGG-9 and 3.5 ± 0.7 × 10⁵ M _⊙ for MGG-11 (where the quoted errors include σ _r, r_hp 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.