Bulges versus disks: the evolution of angular momentum in cosmological simulations of galaxy formation

TL;DR

This study uses cosmological simulations to compare the angular momentum evolution in bulge- and disk-dominated galaxies, highlighting the role of feedback and star formation in preserving angular momentum for disk formation.

Contribution

It demonstrates how different baryonic physics assumptions influence angular momentum retention, supporting classical disc formation theory through simulation analysis.

Findings

Disk-dominated galaxy's angular momentum closely follows dark matter halo evolution.

Bulge-dominated galaxy loses 90% of its angular momentum during mergers.

Feedback processes prevent early angular momentum loss, enabling disk formation.

Abstract

We investigate the evolution of angular momentum in simulations of galaxy formation in a cold dark matter universe. We analyse two model galaxies generated in the N-body/hydrodynamic simulations of Okamoto et al. Starting from identical initial conditions, but using different assumptions for the baryonic physics, one of the simulations produced a bulge-dominated galaxy and the other one a disc-dominated galaxy. The main difference is the treatment of star formation and feedback, both of which were designed to be more efficient in the disc-dominated object. We find that the specific angular momentum of the disc-dominated galaxy tracks the evolution of the angular momentum of the dark matter halo very closely: the angular momentum grows as predicted by linear theory until the epoch of maximum expansion and remains constant thereafter. By contrast, the evolution of the angular momentum of the bulge-dominated galaxy resembles that of the central, most bound halo material: it also grows at first according to linear theory, but 90% of it is rapidly lost as pre-galactic fragments, into which gas had cooled efficiently, merge, transferring their orbital angular momentum to the outer halo by tidal effects. The disc-dominated galaxy avoids this fate because the strong feedback reheats the gas which accumulates in an extended hot reservoir and only begins to cool once the merging activity has subsided. Our analysis lends strong support to the classical theory of disc formation whereby tidally torqued gas is accreted into the centre of the halo conserving its angular momentum.