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 thirteen nearby galaxies selected from a volume-limited sample from the fifth data release of the Sloan Digital Sky Survey (SDSS), from which about 580 young (< 20 Myr), massive (>3 x 104 Msun) clusters were selected using an automated source extraction routine. The extinctions, ages, and masses were determined by comparing their ugiz magnitudes to those generated from starburst models. Completeness corrections were performed using Monte Carlo simulations in which artificial clusters were inserted into each galaxy. Foreground stellar and background galactic contamination was assessed by analyzing SDSS images of fields around the sample galaxies and found to be small. 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 >105 Msun are statistically indistinguishable. For clusters more massive than 105 Msun, the ICMF of the irregular galaxies is reasonably well fit by a power law dN(M)/dM} ~ M-a with a = 1.62 +/- 0.08 Similar results were obtained for the ICMF of the spiral galaxy sample, with a = 1.74 +/- 0.07. We discuss the implications of our result for theories of star cluster formation, which appears to be independent of metallicity and galactic shear rate. We examine the evolution of visual extinction, AV, with cluster age and find significant reduction in median extinction after ~5-10 Myr by about 0.5 mag for clusters in both spiral and irregular galaxies. We identify this timescale as that required for young, massive star clusters to disperse or move away from their natal gas.
Here is the full version of Table 2 from the above in two formats. The first is the (compressed) raw TeX file and the second is a archive of machine-readable tables for each galaxy.
| Table 2 | ||
| tab2.tex.gz | TeX version | 48K |
| tab2.tar.gz | Machine-readable archive | 44K |