We present high dispersion echelle spectroscopy of the very luminous , young super star cluster ( SSC ) ‘ F ’ in M82 , obtained with the 4.2-m William Herschel Telescope ( WHT ) , for the purpose of deriving its dynamical mass and assessing whether it will survive to become an old globular cluster .
In addition to the stellar lines , the spectrum contains complex Na I absorption and broad emission lines from the ionized gas .
We measure a stellar velocity dispersion of 13.4 \pm 0.7 km s ^ { -1 } , a projected half-light radius of 2.8 \pm 0.3 pc from archival HST/WFPC2 images , and derive a dynamical mass of 1.2 \pm 0.1 \times 10 ^ { 6 } M _ { \odot } , demonstrating that M82-F is a very massive , compact cluster .
We determine that the current luminosity-to-mass ratio ( L _ { V } / M ) _ { \odot } for M82-F is 45 \pm 13 .
Comparison with spectral synthesis models shows that ( L _ { V } / M ) _ { \odot } is a factor of \sim 5 higher than that predicted for a standard Kroupa ( 2001 ) initial mass function ( IMF ) at the well-determined age for M82-F of 60 \pm 20 Myr .
This high value of ( L _ { V } / M ) _ { \odot } indicates a deficit of low mass stars in M82-F ; the current mass function ( MF ) evidently is ‘ top-heavy ’ .
We find that a lower mass cutoff of 2–3 M _ { \odot } is required to match the observations for a MF with a slope \alpha = 2.3. Since the cluster apparently lacks long-lived low mass stars , it will not become an old globular cluster , but probably will dissolve at an age of \leq 2 Gyr .
We also derive up-dated luminosity-to-mass ratios for the younger SSCs NGC 1569A and NGC 1705-1 .
For the first object , the observations are consistent with a slightly steeper MF ( \alpha = 2.5 ) whereas for NGC 1705-1 , the observed ratio requires the MF to be truncated near 2 M _ { \odot } for a slope of \alpha = 2.3 .
We discuss the implications of our findings in the context of large scale IMF variations ; with the present data the top-heavy MF could reflect a local mass segregation effect during the birth of the cluster .
M82-F likely formed in a dense molecular cloud ; however , its high radial velocity with respect to the centre of M82 ( \sim - 175 km s ^ { -1 } ) suggests it is on an eccentric orbit and now far from its birthplace , so the environment of its formation is unknown .