Astronomy & Astrophysics

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Issue		A&A Volume 446, Number 2, February I 2006
Page(s)		L9 - L12
Section		Letters
DOI		10.1051/0004-6361:200500224

A&A 446, L9-L12 (2006)
DOI: 10.1051/0004-6361:200500224

Letter

Observational evidence for a truncation of the star cluster initial mass function at the high mass end

M. Gieles¹, S. S. Larsen², R. A. Scheepmaker¹, N. Bastian³, M. R. Haas¹ and H. J. G. L. M. Lamers^{1, 4}

¹ Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, The Netherlands
e-mail: gieles@astro.uu.nl
² European Astronomical Institute, Karl-Schwarzchild-Strasse 2, 85748 Garching b. Munchen, Germany
³ Department of Physics and Astronomy, University College London, Gower Street, London, WC1E 6BT, UK
⁴ SRON Laboratory for Space Research, Sorbonnelaan 2, 3584 CA Utrecht, The Netherlands

(Received 13 September 2005 / Accepted 8 December 2005 )

Abstract
We present the luminosity function (LF) of star clusters in M 51 based on HST/ACS observations taken as part of the Hubble Heritage project. The clusters are selected based on their size and with the resulting 5990 clusters we present one of the largest cluster samples of a single galaxy. We find that the LF can be approximated with a double power-law distribution with a break around M_V = -8.9. On the bright side the index of the power-law distribution is steeper ( $\alpha = 2.75$ ) than on the faint-side ( $\alpha = 1.93$ ), similar to what was found earlier for the "Antennae" galaxies. The location of the bend, however, occurs about 1.6 mag fainter in M 51. We confront the observed LF with the model for the evolution of integrated properties of cluster populations of Gieles et al. (2006, A&A, accepted), which predicts that a truncated cluster initial mass function would result in a bend in, and a double power-law behaviour of, the integrated LF. The combination of the large field-of view and the high star cluster formation rate of M 51 make it possible to detect such a bend in the LF. Hence, we conclude that there exists a fundamental upper limit to the mass of star clusters in M 51. Assuming a power-law cluster initial mass function with exponentional cut-off of the form $N\, {\rm d} M \propto M^{-\beta}\,\exp(-M/M_{\rm C})\, {\rm d} M$ , we find that $M_{\rm C} = 10^5\, M_{\odot}$ . A direct comparison with the LF of the "Antennae" suggests that there $M_{\rm C} = 4\times10^5\, M_{\odot}$ .

Key words: galaxies: evolution -- galaxies: individual: M 51 -- galaxies: star clusters -- globular clusters: general

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