  Sub-virial gravitational collapse is one mechanism by which star clusters may
form. Here we investigate whether this mechanism can be inferred from
observations of young clusters. To address this question, we have computed SPH
simulations of the initial formation and evolution of a dynamically young star
cluster through cold (sub-virial) collapse, starting with an ellipsoidal,
turbulently seeded distribution of gas, and forming sink particles representing
(proto)stars. While the initial density distributions of the clouds do not have
large initial mass concentrations, gravitational focusing due to the global
morphology leads to cluster formation. We use the resulting structures to
extract observable morphological and kinematic signatures for the case of
sub-virial collapse. We find that the signatures of the initial conditions can
be erased rapidly as the gas and stars collapse, suggesting that kinematic
observations need to be made either early in cluster formation and/or at larger
scales, away from the growing cluster core. Our results emphasize that a
dynamically young system is inherently evolving on short timescales, so that it
can be highly misleading to use current-epoch conditions to study aspects such
as star formation rates as a function of local density. Our simulations serve
as a starting point for further studies of collapse including other factors
such as magnetic fields and stellar feedback.
