The observed mass function of young star clusters

I will review observational constraints on the mass function of young
star clusters (the "Initial Cluster Mass Function"; ICMF). Direct fits to 
sufficiently large samples of young star clusters suggest that the ICMF is 
well approximated by a Schechter function with low-mass slope dN/dM ~ M^-2 
and an exponential cut-off at a few 100,000 Msun in quiescent environments 
(spiral discs). In starburst environments the cut-off appears to be at 
significantly higher masses, at least several million Msun, similar to old globular 
cluster systems.  A Schechter ICMF is also consistent with observations of 
luminosity functions of cluster populations in spiral galaxies, that typically are steeper 
than dN/dL ~ L^-2, and with the observed correlation between age and luminosity 
of the brightest clusters in galaxies. The cut-off observed in the ICMF may be related 
to the upper mass limit of Giant Molecular Clouds which occurs at several million Msun, 
consistent with a low overall cluster formation efficiency within GMCs in spiral
discs. Based on comparison with other spiral galaxies, the Milky Way should host
about 20 clusters with masses > 10^5 Msun and ages < 200 Myr.