Angular momentum generation in cold gravitational collapse
  During the violent relaxation of a self-gravitating system a significant
fraction of its mass may be ejected. If the time varying gravitational field
also breaks spherical symmetry this mass can potentially carry angular
momentum. Thus starting initial configurations with zero angular momentum can
in principle lead to a bound virialized system with non-zero angular momentum.
We explore here, using numerical simulations, how much angular momentum can be
generated in a virialized structure in this way, starting from configurations
of cold particles which are very close to spherically symmetric. For initial
configurations in which spherical symmetry is broken only by the Poissonian
fluctuations associated with the finite particle number $N$, with $N$ in range
$10^3$ to $10^5$, we find that the relaxed structures have standard "spin"
parameters $\lambda \sim 10^{-3}$, and decreasing slowly with $N$. For slightly
ellipsoidal initial conditions, in which the finite-$N$ fluctuations break the
residual reflection symmetries, we observe values $\lambda \sim 10^{-2}$, of
the same order of magnitude as those reported for elliptical galaxies. The net
angular momentum vector is typically aligned close to normal to the major
semi-axis of the triaxial relaxed structure, and also with that of the ejected
mass. This simple mechanism may provide an alternative, or complement, to
"tidal torque theory" for understanding the origin of angular momentum in
astrophysical structures.
