Cluster mass dependent truncation of the upper IMF: evidence from
  observations and simulations
  We attempt to evaluate whether the integrated galactic IMF (IGIMF) is
expected to be steeper than the IMF within individual clusters through direct
evaluation of whether there is a systematic dependence of maximum stellar mass
on cluster mass. We show that the result is sensitive to observational
selection biases and requires an accurate knowledge of cluster ages,
particularly in more populous clusters. At face value there is no compelling
evidence for non-random selection of stellar masses in low mass clusters but
there is arguably some evidence that the maximum stellar mass is anomalously
low (compared with the expectations of random mass selection) in clusters
containing more than several thousand stars. Whether or not this effect is then
imprinted on the IGIMF then depends on the slope of the cluster mass function.
We argue that a more economical approach to the problem would instead involve
direct analysis of the upper IMF in clusters using statistical tests for
truncation of the mass function. When such an approach is applied to data from
hydrodynamic simulations we find evidence for truncated mass functions even in
the case of simulations without feedback.
