Angular momentum generation in cold gravitational collapse

TL;DR

This paper investigates how cold gravitational collapse can generate angular momentum in virialized structures starting from near-spherical initial conditions, using numerical simulations to explore the resulting spin parameters.

Contribution

It demonstrates that gravitational collapse can produce measurable angular momentum without initial rotation, offering an alternative to tidal torque theory.

Findings

Relaxed structures have spin parameters ~10^{-3} for spherical initial conditions.

Ellipsoidal initial conditions yield higher spin parameters ~10^{-2}.

Angular momentum vectors align with the structure's major axis and ejected mass.

Abstract

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.