SCYON Abstract

Received on August 5 2002

Star Cluster Formation and Disruption Time-Scales -- II. Evolution of the Star Cluster System in M82's Fossil Starburst

Authors	Richard de Grijs (¹), Nate Bastian (²), Henny J.G.L.M. Lamers (²)
Affiliation	¹ Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge CB3 0HA, UK ² Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
Submitted to	Monthly Notices of the Royal Astronomical Society
Contact	grijs@ast.cam.ac.uk
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Abstract

We obtain new age and mass estimates for the star clusters in M82's fossil starburst region B, based on improved fitting methods. Our new age estimates confirm the peak in the age histogram attributed to the last tidal encounter with M81; we find a peak formation epoch at slightly older ages than previously published, log (t_peak / yr) = 9.04, with a Gaussian sigma of Delta log( t_width) = 0.273. The actual duration of the burst of cluster formation may have been shorter because uncertainties in the age determinations may have broadened the peak. Our improved mass estimates confirm that the (initial) masses of the M82 B clusters with V < 22.5 mag are mostly in the range 10⁴ - 10⁶ Mo, with a median mass of M_cl = 1.08 x 10⁵ Mo. The formation history of the observed clusters shows a steady decrease towards older ages. This indicates that cluster disruption has removed a large fraction of the older clusters.

Adopting the expression for the cluster disruption time-scale of dis(M)= disref (M/10⁴ M_o)^gamma with gamma ~0.62 (Paper I), we find that the ratios between the real cluster formation rates in the pre-burst phase (log(t/yr) > 9.4), the burst-phase (8.4 < log(t/yr) < 9.4) and the post-burst phase (log(t/yr) < 8.4) are about 1:2:1/40. The formation rate during the burst may have been higher if the actual duration of the burst was shorter than adopted.

The mass distribution of the clusters formed during the burst shows a turnover at log(M_cl/M_o) ~5.3 which is not caused by selection effects. This distribution can be explained by cluster formation with an initial power-law mass function of slope alpha=2 up to a maximum cluster mass of M_max = 3 x 10⁶ M_o, and cluster disruption with a normalisation time-scale disref / burst=(3.0 ± 0.3) x 10^-2. For a burst age of 1 x 10⁹ yr, we find that the disruption time-scale of a cluster of 10⁴ M_o is disref ~ 3 x 10⁷ years, with an uncertainty of approximately a factor of two. This is the shortest disruption time-scale known in any galaxy.