M 82 - F : A Doomed Super Star Cluster ? ⋆

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± 0.7 kms, a projected half-light radius of 2.8 ± 0.3 pc from archival HST/WFPC2 images, and derive a dynamical mass of 1.2±0.1×10 M⊙, demonstrating that M82-F is a very massive, compact cluster. We determine that the current luminosity-to-mass ratio (LV /M)⊙ for M82-F is 45 ± 13. Comparison with spectral synthesis models shows that (LV /M)⊙ is a factor of ∼ 5 higher than that predicted for a standard Kroupa (2001) initial mass function (IMF) at the well-determined age for M82-F of 60 ± 20 Myr. This high value of (LV /M)⊙ 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 2–3M⊙ is required to match the observations for a MF with a slope α = 2.3. Since the cluster apparently lacks longlived low mass stars, it will not become an old globular cluster, but probably will dissolve at an age of 62 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 (α = 2.5) whereas for NGC 1705-1, the observed ratio requires the MF to be truncated near 2 M⊙ for a slope of α = 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 (∼ −175 km s) suggests it is on an eccentric orbit and now far from its birthplace, so the environment of its formation is unknown.