  The initial cluster mass function (ICMF) is a fundamental property of star
formation in galaxies. To gauge its universality, we measure and compare the
ICMFs in irregular and spiral galaxies. Our sample of irregular galaxies is
based on 13 nearby galaxies selected from a volume-limited sample from the
fifth data release of the Sloan Digital Sky Survey (SDSS). The extinctions,
ages, and masses were determined by comparing their u'g'i'z' magnitudes to
those generated from starburst models. Completeness corrections were performed
using Monte Carlo simulations in which artificial clusters were inserted into
each galaxy. We analyzed three nearby spiral galaxies with SDSS data in exactly
the same way to derive their ICMF based on a similar number of young, massive
clusters as the irregular galaxy ICMF. We find that the ICMFs of irregular and
spiral galaxies for masses >3x10^4 M_sun are statistically indistinguishable.
For clusters more massive than 3x10^4 M_sun, the ICMF of the irregular galaxies
is reasonably well fit by a power law dN(M)/dM ~ M^-a_M with a_M = 1.88 +/-
0.09. Similar results were obtained for the ICMF of the spiral galaxy sample
but with a_M = 1.75 +/- 0.06. We discuss the implications of our result for
theories of star cluster formation: the shape of the ICMF appears to be
independent of metallicity and galactic shear rate.
