SIMBAD references

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), from which about 320 young (≤20 Myr), massive (≳3x10⁴M_☉) clusters or associations were selected using an automated source extraction routine. 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. Foreground stellar and background galactic contaminations were 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 exactly in 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 >10^4.4M_☉ are statistically indistinguishable. For clusters and associations more massive than 10^4.4M_☉, the ICMF of the irregular galaxies is reasonably well fit by a power law with α_M= 1.88±0.09. Similar results were obtained for the ICMF of the spiral galaxy sample but with α_M = 1.75±0.06. We discuss the implications of our results on theories of star cluster formation, which appear to indicate that the power-law indices are independent of metallicity and galactic shear rate. We also examine the evolution of visual extinction, A_V, 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 discuss the implications of our results for theories of star cluster formation: the shape of the ICMF appears to be independent of metallicity and galactic shear rate.