The Properties of Young Clusters in M82

Abstract

We present a detailed study of two luminous super star clusters in the starburst galaxy M82. Spectra, covering 3250-8790 A at a resolution of 1.6 A, were obtained at the 4.2 m William Herschel Telescope (WHT) for cluster F and the highly reddened cluster L. We compare the strengths of the observed Balmer absorption lines and the Balmer jump in the blue spectrum of F with theoretical model cluster spectra using the PEGASE spectral synthesis code to derive an age of 60+/-20 Myr. For cluster L, we find that the similarities in the strength of the Ca II triplet and overall spectral appearance with cluster F suggest a similar age. The brightness and compactness of cluster F make it an ideal candidate for determining its dynamical mass by measuring the velocity dispersion. We present the results of such an investigation based on echelle spectra at a resolution of 8 km/s obtained at the WHT from 5760-9140 A. By cross-correlating various wavelength regions in the spectrum of cluster F with cool giant and supergiant template stars, we derive a velocity dispersion and, by application of the virial theorem, determine a dynamical mass of 2 million solar masses. We compare our derived mass with those determined for other young super star clusters and discuss whether our derived parameters are consistent with cluster F being able to survive to become a globular cluster.