On the Interpretation of the Age Distribution of Star Clusters in the Small Magellanic Cloud

• Published in: The Astrophysical journal Vol. 668; no. 1; pp. 268 - 274
• Main Authors: Gieles, Mark, Lamers, Henny J. G. L. M, Portegies Zwart, Simon F
• Format: Journal Article
• Language: English
• Published: Chicago, IL IOP Publishing 10-10-2007; University of Chicago Press
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; Galaxy clusters; stars: formation; galaxies: individual (Small Magellanic Cloud); Star formation; Galaxies; Small Magellanic Cloud; galaxies: star clusters; Star clusters; Age
• ISSN: 0004-637X; 1538-4357
• DOI: 10.1086/520795

We reanalyze the age distribution (dN/dt) of star clusters in the Small Magellanic Cloud (SMC) using age determinations based on the Magellanic Cloud Photometric Survey. For ages younger than 3 x [unk] yr the dN/dt distribution can be approximated by a power-law distribution, dN/dt proportional to [unk] super(-) beta , with - beta = -0.70 plus or minus 0.05 or - beta = -0.84 plus or minus 0.04, depending on the model used to derive the ages. Predictions for a cluster population without dissolution limited by a V-band detection result in a power-law dN/dt distribution with an index of similar to -0.7. This is because the limiting cluster mass increases with age, due to evolutionary fading of clusters, reducing the number of observed clusters at old ages. When a mass cut well above the limiting cluster mass is applied, the dN/dt distribution is flat up to 1 Gyr. We conclude that cluster dissolution is of small importance in shaping the dN/dt distribution, and incompleteness causes dN/dt to decline. The reason that no (mass independent) infant mortality of star clusters around similar to 10-20 Myr is found is explained by a detection blas toward clusters without nebular emission, i.e., clusters that have survived the infant mortality phase. The reason we find no evidence for tidal (mass dependent) cluster dissolution in the first gigayear is explained by the weak tidal field of the SMC. Our results are in sharp contrast to the interpretation of Chandar et al., who interpret the declining dN/dt distribution as rapid cluster dissolution. This is due to their erroneous assumption that the sample is limited by cluster mass, rather than luminosity.