Cluster Mass Functions in the Large and Small Magellanic Clouds: Fading and Size-of-Sample Effects

Abstract

The properties of $939 star clusters in the Large and Small Magellanic Clouds were determined from ground-based CCD images in UBVR passbands. The areal coverage was extensive, corresponding to 11.0 kpc 2 in the LMC and 8.3 kpc 2 in the SMC. After corrections for reddening, the colors and magnitudes of the clusters were converted to ages and masses, and the resulting mass distributions were searched for the effects of fading, evaporation, and size-of-sample bias. The data show a clear signature of cluster fading below the detection threshold. The initial cluster mass function (ICMF) was determined by fitting the mass and age distributions with cluster population models. These models suggest a new method to determine the ICMF that is nearly independent of fading or disruption and is based on the slope of a correlation between age and the maximum cluster mass in equally spaced intervals of log age. For a nearly uniform star formation rate, this correlation has a slope equal to 1/( À 1) for an ICMF of dn(M)/dM / M À . We determine that is between 2 and 2.4 for the LMC and SMC using this method plus another method in which models are fitted to the mass distribution integrated over age and to the age distribution integrated over mass. The maximum mass method also suggests that the cluster formation rate in the LMC age gap between 3 and 13 Gyr is about a factor of 10 below that in the period from 0.1 to 1 Gyr. The oldest clusters correspond in age and mass to halo globular clusters in the Milky Way. They do not fit the trends for lower mass clusters but appear to be a separate population that either had a very high star formation rate and became depleted by evaporation or formed with only high masses.

Modeling the ionizing spectra of H II regions: individual stars versus stellar ensembles

“…This representation of the ICMF was proposed by Lada & Lada (2003) for clusters with masses between ∼50 and 10 3 M . The application of this ICMF to the complete mass range used here is supported by the studies of Zhang & Fall (1999) and Hunter et al (2003).…”

Astrophysical properties of star clusters in the Magellanic Clouds homogeneously estimated by ASteCA

“…We estimate average E B−V values for the S/LMC of 0.03 ± 0.03 mag and 0.05 ± 0.05 mag. These estimates are approximately a third of those used, for example, in the Hunter et al (2003) study because our sample does not contain clusters in the regions of the MCs most affected by dust.…”

Galactic‐Field Initial Mass Functions of Massive Stars

“…The mass distribution of local embedded clusters is a power law, ecl / M À ecl , with % 2 for 20dM ecl =M d1100 (Lada & Lada 2003). Hunter et al (2003) find = 2-2.4 for the initial cluster mass function in the Large and Small Magellanic Clouds, and Zhang & Fall (1999) report ¼ 2 AE 0:08 for young star clusters with 10 4 dM ecl =M d10 6 in the Antennae galaxies. The consensus thus emerges that the cluster mass function (CMF) of young star clusters of all masses can be described by a single power-law form with % 2, which is consistent with the star-cluster samples in six nearby spiral galaxies studied by Larsen (2002) as well as the MF of young globular clusters (Richtler 2003).…”