The Causes of Halo Shape Changes Induced by Cooling Baryons: Disks Versus Substructures

TL;DR

This study investigates how baryonic condensation affects dark matter halo shapes, finding that disk growth alone does not cause irreversible shape changes unless angular momentum transfer occurs, which can significantly alter halo structure.

Contribution

It demonstrates that baryonic growth does not permanently change halo triaxiality unless angular momentum is transferred, challenging previous assumptions about baryon-induced shape transformations.

Findings

Halo becomes rounder with disk growth, but reverts after disk evaporation.

Box orbits are not destroyed but become rounder with potential changes.

Angular momentum transfer causes irreversible halo shape changes.

Abstract

Cold dark matter cosmogony predicts triaxial dark matter halos, whereas observations find quite round halos. This is most likely due to the condensation of baryons leading to rounder halos. We examine the halo phase space distribution basis for such shape changes. Triaxial halos are supported by box orbits, which pass arbitrarily close to the density center. The decrease in triaxiality caused by baryons is thought to be due to the scattering of these orbits. We test this hypothesis with simulations of disks grown inside triaxial halos. After the disks are grown we check whether the phase space structure has changed by evaporating the disks and comparing the initial and final states. While the halos are substantially rounder when the disk is at full mass, their final shape after the disk is evaporated is not much different from the initial. Likewise, the halo becomes (more) radially anisotropic when the disk is grown, but the final anisotropy is consistent with the initial. Only if the baryons are unreasonably compact or massive does the halo change irreversibly. We show that the character of individual orbits is not generally changed by the growing mass. Thus the central condensation of baryons does not destroy enough box orbits to cause the shape change. Rather, box orbits merely become rounder along with the global potential. However, if angular momentum is transferred to the halo, either via satellites or via bars, a large irreversible change in the halo distribution occurs. The ability of satellites to alter the phase space distribution of the halo is of particular concern to galaxy formation simulations since halo triaxiality can profoundly influence the evolution of disks.