Similarities in Populations of Star Clusters
  We compare the observed mass functions and age distributions of star clusters
in six well-studied galaxies: the Milky Way, Magellanic Clouds, M83, M51, and
Antennae. In combination, these distributions span wide ranges of mass and age:
$10^2\lea M/M_{\odot}\lea10^6$ and $10^6\lea\tau/yr \lea10^9$. We confirm that
the distributions are well represented by power laws: $dN/dM\propto M^{\beta}$
with $\beta \approx-1.9$ and $dN/d\tau\propto\tau^{\gamma}$ with $\gamma\approx
-0.8$. The mass and age distributions are approximately independent of each
other, ruling out simple models of mass-dependent disruption. As expected,
there are minor differences among the exponents, at a level close to the true
uncertainties, $\epsilon_{\beta}\sim\epsilon_{\gamma}\sim$~0.1--0.2. However,
the overwhelming impression is the similarity of the mass functions and age
distributions of clusters in these different galaxies, including giant and
dwarf, quiescent and interacting galaxies. This is an important empirical
result, justifying terms such as "universal" or "quasi-universal." We provide a
partial theoretical explanation for these observations in terms of physical
processes operating during the formation and disruption of the clusters,
including star formation and feedback, subsequent stellar mass loss, and tidal
interactions with passing molecular clouds. A full explanation will require
additional information about the molecular clumps and star clusters in galaxies
beyond the Milky Way.
