  Star clusters in 6 nearby spiral galaxies are examined using archive images
from HST/WFPC2. The galaxies have previously been studied from the ground and
some of them are known to possess rich populations of "young massive clusters"
(YMCs). Comparison with the HST images indicates a success-rate of about 75%
for the ground-based cluster detections, with typical contaminants being blends
or loose groupings of several stars in crowded regions. The luminosity
functions (LFs) of cluster candidates identified on the HST images are analyzed
and compared with existing data for the Milky Way and the LMC. The LFs are well
approximated by power-laws of the form dN(L)/dL ~ L^alpha, with slopes in the
range -2.4<alpha<-2.0. The steeper slopes tend to be found among fits covering
brighter magnitude intervals, although direct hints of a variation in the LF
slope with magnitude are seen only at low significance in two galaxies. The
surface density of star clusters at a reference magnitude of M(V)=-8 scales
with the mean star formation rate per unit area, Sigma(SFR). Assuming that the
LF can be generally expressed as a power-law with normalization proportional to
the galaxy area (A) and Sigma(SFR), the maximum cluster luminosity expected in
a galaxy from random sampling of the LF is estimated as a function of
Sigma(SFR) and A. The predictions agree well with existing observations of
galaxies spanning a wide range of Sigma(SFR) values, suggesting that sampling
statistics play an important role in determining the maximum observed
luminosities of young star clusters in galaxies.
