RADIAL ORBIT INSTABILITY IN A HUBBLE-EXPANDING UNIVERSE

We investigate the role of the radial orbit instability in the formation of galactic halos through dissipationless collapse. We present numerical simulations where both the Hubble expansion and the inhomogeneities of the protogalactic material have been taken into account. We find that the radial orbit instability is a transient effect for a wide variety of initial conditions, including those with cosmological-like density fluctuations, as a result of the combination of two effects: the inhomogeneities give rise to condensations whose interactions introduce tangential motions, and the Hubble expansion delays the collapse long enough for those motions to grow, preventing the instability. We conclude that, with reasonable initial conditions, the end product of a dissipationless collapse does not need to be a triaxial system.