Infant mortality in the hierarchical merging scenario: Dependency on gas
  expulsion timescales
  We examine the effects of gas expulsion on initially sub-structured and
out-of-equilibrium star clusters. We perform N-body simulations of the
evolution of star clusters in a static background potential before adjusting
that potential to model gas expulsion. We investigate the impact of varying the
rate at which the gas is removed, and the instant at which gas removal begins.
Reducing the rate at which the gas is expelled results in an increase in
cluster survival. Quantitatively, this dependency is approximately in agreement
with previous studies, despite their use of smooth, and virialised initial
stellar distributions. However, the instant at which gas expulsion occurs is
found to have a strong effect on cluster response to gas removal. We find if
gas expulsion occurs prior to one crossing time, cluster response is poorly
described by any global parameters. Furthermore in real clusters the instant of
gas expulsion is poorly constrained. Therefore our results emphasis the highly
stochastic and variable response of star clusters to gas expulsion.
