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.Comment: 20 pages, 4 figures, 2 tables; published in the Astrophysical Journal, 752:96 (2012 June 20

The Age Distribution of Massive Star Clusters in the Antennae Galaxies

We determine the age distribution of star clusters in the Antennae galaxies (NGC 4038/9) for two mass-limited samples (M > 3 x 10^4 M_{\odot} and M > 2 x 10^5 M_{\odot}). This is based on integrated broadband UBVI and narrowband H-alpha photometry from deep images taken with the Hubble Space Telescope. We find that the age distribution of the clusters declines steeply, approximately as dN/d\tau \propto \tau^{-1}. The median age of the clusters is ~10^7 yr, which we interpret as evidence for rapid disruption ("infant mortality"). It is very likely that most of the young clusters are not gravitationally bound and were disrupted near the times they formed by the energy and momentum input from young stars to the interstellar matter of the protoclusters. At least 20% and possibly all stars form in clusters and/or associations, including those that are unbound and short-lived.Comment: 11 pages, 2 figures. To appear in the ApJ Letters; Submitted 2004 July 29; accepted 2005 August

A Comparison of Methods for Determining the Age Distribution of Star Clusters: Application to the Large Magellanic Cloud

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 found recently for the age distribution chi(t) of clusters in the Large Magellanic Cloud (LMC). We found that chi(t) can be described approximately by a power law chi(t) propto t^{gamma}, with gamma -0.8, by counting clusters in the mass-age plane, i.e., by constructing chi(t) directly from mass-limited samples. Gieles & Bastian inferred a value of gamma~, based on the slope of the relation between the maximum mass of clusters in equal intervals of log t, hereafter the M_max method, 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 chi(t) propto t^-0.8 for t<10^9 yr. The reason for the apparent discrepancy is that our analysis includes many massive (M>1.5x10^3 M_sol), recently formed (t<10^7 yr) clusters, 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, irregular and spiral galaxies, has a similar declining shape. We interpret this approximately "universal" shape as due primarily to the progressive disruption of star clusters over their first ~few x 10^8 yr, starting soon after formation, and discuss some observational and physical implications of this early disruption for stellar populations in galaxies.Comment: 21 pages, 5 figures, published in the Astrophysical Journal, volume 713, page 134

Density Dependence of the Mass Function of Globular Star Clusters in the Sombrero Galaxy and its Dynamical Implications

We have constructed the mass function of globular star clusters in the Sombrero galaxy in bins of different internal half-mass density rho_h and projected galactocentric distance R. This is based on the published measurements of the magnitudes and effective radii of the clusters by Spitler et al. (2006) in BVR images taken with the ACS on HST. We find that the peak of the mass function M_p increases with rho_h by a factor of about 4 but remains nearly constant with R. Our results are almost identical to those presented recently by McLaughlin & Fall (2007) for globular clusters in the Milky Way. The mass functions in both galaxies agree with a simple, approximate model in which the clusters form with a Schechter initial mass function and evolve subsequently by stellar escape driven by internal two-body relaxation. These findings therefore undermine recent claims that the present peak of the mass function of globular clusters must have been built into the initial conditions.Comment: Astrophysical Journal Letters, in press. 4 page

New Tests for Disruption Mechanisms of Star Clusters: The Large and Small Magellanic Clouds

We compare the observed bivariate distribution of masses(M) and ages(t) of star clusters in the LMC with the predicted distributions g(M,t) from 3 idealized models for the disruption of star clusters: (1)sudden mass-dependent disruption;(2)gradual mass-dependent disruption; and (3)gradual mass-independent disruption. The model with mass-{\em in}dependent disruption provides a good, first-order description of these cluster populations, with g(M,t) propto M^{beta} t^{gamma}, beta=-1.8+/-0.2 and gamma=-0.8+/-0.2, at least for clusters with ages t<10^9 yr and masses M<10^3 M_sol (more specifically, t<10^7(M/10^2 M_sol)^{1.3} yr). This model predicts that the clusters should have a power-law luminosity function, dN/dL propto L^-1.8, in agreement with observations. The first two models, on the other hand, fare poorly when describing the observations, refuting previous claims that mass-dependent disruption of star clusters is observed in the LMC over the studied M-t domain. Clusters in the SMC can be described by the same g(M,t) distribution as for the LMC, but with smaller samples and hence larger uncertainties. The successful g(M,t) model for clusters in the Magellanic Clouds is virtually the same as the one for clusters in the merging Antennae galaxies, but extends the domain of validity to lower masses and to older ages. This indicates that the dominant disruption processes are similar in these very different galaxies over at least t<10^8 yr and possibly t<10^9 yr. The mass functions for young clusters in the LMC are power-laws, while that for ancient globular clusters is peaked. We show that the observed shapes of these mass functions are consistent with expectations from the simple evaporation model presented by McLaughlin & Fall.Comment: 46 pages, 17 figures, published ApJ, vol 711, page 126

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