We present the luminosity function ( LF ) of star clusters in M51 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 5 990 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 M51 .
We confront the observed LF with the model for the evolution of integrated properties of cluster populations of Gieles et al .
( 2005b ) , 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 M51 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 M51 .
Assuming a power-law cluster initial mass function with exponentional cut-off of the form N \mbox { $ { d } $ } M \propto M ^ { - \beta } \exp ( - M / M _ { C } ) \mbox { $ { d } $ } M , we find that M _ { C } = 10 ^ { 5 } \mbox { $M _ { \odot } $ } .
A direct comparison with the LF of the “ Antennae ” suggests that there M _ { C } = 4 \times 10 ^ { 5 } \mbox { $M _ { \odot } $ } .