The mass function of young star clusters in spiral galaxies

• Published in: Astronomy and astrophysics (Berlin) 2009-02, Vol.494 (2), p.539-551
• Main Author: Larsen, S. S.
• Format: Article
• Language: English
• Subjects: Astronomy; Earth, ocean, space; Exact sciences and technology; galaxies: spiral; galaxies: star clusters; Galaxy: open clusters and associations: general
• ISSN: 0004-6361; 1432-0746
• DOI: 10.1051/0004-6361:200811212

Aims. The initial cluster mass function (ICMF) in spiral discs is constrained and compared with data for old globular clusters and young clusters in starbursts. Methods. For a given absolute magnitude, the cluster age distribution depends on the ICMF. Here, the behaviour of the median age-magnitude relation is analysed in detail for Schechter ICMFs with various cut-off masses, Mc. The calculated relations are compared with observations of the brightest clusters in spiral galaxies. Schechter functions are also fitted directly to observed mass functions (MFs). Results. A single Schechter ICMF with an Mc of a few times 105 $M_{\odot}$ can reproduce the observed ages and luminosities of the brightest (and 5th brightest) clusters in the spirals if disruption of optically visible clusters is dominated by relatively slow secular evolution. A Schechter function fit to the combined cluster MF for all spirals in the sample yields Mc = (2.1±0.4)$\times$105 $M_{\odot}$. The MFs in cluster-poor and cluster-rich spirals are statistically indistinguishable. An Mc = 2.1$\times$105 $M_{\odot}$ Schechter function also fits the MF of young clusters in the Large Magellanic Cloud. If the same ICMF applies in the Milky Way, a bound cluster with M > 105 $M_{\odot}$ will form about once every 107 years, while an M > 106 $M_{\odot}$ cluster will form only once every 50 Gyr. Luminosity functions (LFs) of model cluster populations drawn from an Mc = 2.1$\times$105 $M_{\odot}$ Schechter ICMF generally agree with LFs observed in spiral galaxies. Conclusions. The ICMF in present-day spiral discs can be modelled as a Schechter function with Mc ≈ 2$\times$105 $M_{\odot}$. However, the presence of significant numbers of M > 106 $M_{\odot}$ (and even M > 107 $M_{\odot}$) clusters in some starburst galaxies makes it unlikely that the Mc value derived for spirals is universal. In high-pressure environments, such as those created by complex gas kinematics and feedback in mergers, Mc can shift to higher masses than in quiescent discs.