Star cluster formation and disruption time-scales - II. Evolution of the star cluster system in the fossil starburst of M82

Abstract

We obtain new age and mass estimates for the star clusters in the fossil starburst region B of M82, 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(tpeak yr-1) = 9.04, with a Gaussian σ of Δ log(twidth) = 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 M⊙, with a median mass of Mcl = 1.08 × 105 M⊙. 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 tdis(M) = t4dis (M/10 4M⊙)γ with γ ≃ 0.62 (Paper I), we find that the ratios between the real cluster formation rates in the pre-burst phase [log(t yr-1) ≥ 9.4], the burst phase [8.4 < log(t yr -1 < 9.4] and the post-burst phase [log(t yr-1) ≤ 8.4] are approximately 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(Mcl/M⊙) ≃ 5.3 that is not caused by selection effects. This distribution can be explained by cluster formation with an initial power-law mass function of slope α = 2 up to a maximum cluster mass of Mmax = 3 × 106 M⊙ and cluster disruption with a normalization time-scale t4dis/t burst = (3.0 ± 0.3) ± 10-2. For a burst age of 1 × 109 yr, we find that the disruption time-scale of a cluster of 104 M⊙ is t4dis ∼ 3 × 107 yr, with an uncertainty of approximately a factor of 2. This is the shortest disruption time-scale known in any (disc region of a) galaxy.