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