Evolution of Shape in the Field

The evolution in shape of an isolated density enhancement in the early Universe is studied through numerical simulations. The formation scenarios of a cold dissipationless collapse and that of a slow accumulation of gas in a dark-matter halo are examined. In the later case the conversion of gas to stars and the accompanying energy feedback from stellar winds and SNe II are taken into account. It is found that with the more realistic initial conditions the radial orbit instability (ROI) is able to operate at higher values of the virial coefficient (q greater than or equal to 0.3) than previously believed. In the cold dissipationless collapse the system becomes fully axisymmetric on a timescale that is less than one collapse time. A prolate or oblate figure is obtained for low and high rotations, respectively. The inclusion of gas and star formation leads to halos that are more triaxial, while the stellar systems that form in them are oblate, independently of the initial rotation.