Using high-resolution ( R \sim 22,000 ) near-infrared ( 1.51 – 1.75 \mu m ) spectra from Keck Observatory , we measure the kinematic masses of two super star clusters ( SSCs ) in M82 .
Cross-correlation of the spectra with template spectra of cool evolved stars gives stellar velocity dispersions of \sigma _ { r } = 15.9 \pm 0.8 km s ^ { -1 } for J0955505+694045 ( ‘ MGG-9 ’ ) and \sigma _ { r } = 11.4 \pm 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 HST/NICMOS images give half-light radii of r _ { hp } = 2.6 \pm 0.4 pc for MGG-9 and r _ { hp } = 1.2 \pm 0.17 pc for MGG-11 .
Applying the virial theorem , we determine masses of 1.5 \pm 0.3 \times 10 ^ { 6 } M _ { \odot } for MGG-9 and 3.5 \pm 0.7 \times 10 ^ { 5 } M _ { \odot } for MGG-11 ( where the quoted errors include \sigma _ { r } , r _ { hp } and the distance ) .
Population synthesis modelling suggests that MGG-9 is consistent with a standard initial mass function , 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 .