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".
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.