SCYON Abstract

Received on February 27 2006

On the Mass of Dense Star Clusters in Starburst Galaxies from Spectro-Photometry

Authors	J.-J. Fleck(¹), C.M. Boily(¹), A. Lancon(¹) and S. Deiters(²)
Affiliation	(¹) Observatoire astronomique, 11 rue de l'Université, F-67000 Strasbourg, France (²) School of Mathematics, University of Edinburgh, King's Buildings, Edinburgh EH9 3JZ, Scotland, UK
Submitted to	Monthly Notices of the Royal Astronomical Society
Contact	jfleck@astro.u-strasbg.fr
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Abstract

The mass of unresolved young star clusters derived from spectro-photometric data may well be off by a factor of 2 or more once the migration of massive stars driven by mass segregation is accounted for. We quantify this effect for a large set of cluster parameters, including variations in the stellar IMF, the intrinsic cluster mass, and mean mass density. Gas-dynamical models coupled with the Cambridge stellar evolution tracks allow us to derive a scheme to recover the real cluster mass given measured half-light radius, one-dimensional velocity dispersion and age. We monitor the evolution with time of the ratio of real to apparent mass through the parameter η. When we compute η for rich star clusters, we find non-monotonic evolution in time when the IMF stretches beyond a critical cutoff mass of 25.5 M(sun). We also monitor the rise of color gradients between the inner and outer volume of clusters: we find trends in time of the stellar IMF power indices overlapping well with those derived for the LMC cluster NGC 1818 at an age of 30 Myr. We argue that the core region of massive Antennae clusters should have suffered from much segregation despite their low ages. We apply these results to a cluster mass function, and find that the peak of the mass distribution would appear to observers shifted to lower masses by as much as 0.2 dex. The star formation rate (SFR) derived for the cluster population is then underestimated by from 20 to 50 per cent.