The Luminosity Function of Star Clusters in 20 Star-Forming Galaxies Based on Hubble Legacy Archive Photometry

Abstract

Luminosity functions have been determined for star cluster populations in 20 nearby (4-30 Mpc), star-forming galaxies based on ACS source lists generated by the Hubble Legacy Archive. Comparisons are made with other recently generated cluster catalogs demonstrating that the HLA-generated catalogs are of similar quality, but in general do not go as deep. A typical cluster luminosity function can be approximated by a power-law, $dN/dL\propto L^{\alpha}$, with an average value for $\alpha$ of $-2.37$ and RMS scatter = 0.18 when using the F814W ("$I$") band. We find that galaxies with high rates of star formation (or equivalently, with the brightest or largest numbers of clusters) have a slight tendency to have shallower values of $\alpha$. In particular, the Antennae galaxy (NGC4038/39), a merging system with a relatively high star formation rate, has the second flattest luminosity function in the sample. A tentative correlation may also be present between Hubble Type and values of $\alpha$, in the sense that later type galaxies (i.e., Sd and Sm) appear to have flatter luminosity functions. Hence, while there do appear to be some weak correlations, the relative similarity in the values of $\alpha$ for a large number of star-forming galaxies suggests that, to first order, the LFs are fairly universal. We examine the bright end of the luminosity functions and find evidence for a downturn, although it only pertains to about 1% of the clusters. Our uniform database results in a small scatter ($\approx$0.4 to 0.5 mag) in the correlation between the magnitude of the brightest cluster ($M_\mathrm{brightest}$) and log of the number of clusters brighter than $M_{I}=-9$ (log N). We also examine the magnitude of the brightest cluster vs. log SFR for a sample including both dwarfs galaxies and ULIRGS.