2010ApJ...713.1343C

The age distribution of star clusters in nearby galaxies plays a crucial role in evaluating the lifetimes and disruption mechanisms of the clusters. Two very different results have been recently found for the age distribution χ(τ) of clusters in the Large Magellanic Cloud (LMC). We found that χ(τ) can be approximately described by a power law χ(τ) {vprop} τ^γ, with γ ≈ -0.8, by counting clusters in the mass-age plane, i.e., by constructing χ(τ) directly from mass-limited samples. Gieles & Bastian inferred a value of γ ≈ 0, based on the slope of the relation between the maximum mass of clusters in equal intervals of log τ, hereafter, the M_maxmethod, an indirect technique that requires additional assumptions about the upper end of the mass function. However, our own analysis shows that the M_max method gives a result consistent with our direct counting method for clusters in the LMC, namely, χ(τ) {vprop} τ^–0.8 for τ ≲ 10⁹ yr. The reason for the apparent discrepancy is that our analysis includes many intermediate and high-mass clusters (M > 1.5x10³ M_☉), which formed recently (τ ≲ 10⁷ yr), and which are known to exist in the LMC, whereas Gieles & Bastian are missing such clusters. We compile recent results from the literature showing that the age distribution of young star clusters in more than a dozen galaxies, including dwarf and giant galaxies, isolated and interacting galaxies, and irregular and spiral galaxies, has a similar declining shape. We interpret this approximately "universal" shape as primarily due to the progressive disruption of star clusters over their first ∼fewx10⁸ yr, starting soon after formation, and discuss some observational and physical implications of this early disruption for stellar populations in galaxies.