Galactic angular momentum in cosmological zoom-in simulations. I. Disk and bulge components and the galaxy--halo connection

TL;DR

This study uses cosmological zoom-in simulations to analyze the angular momentum evolution of disk galaxies, revealing how their components relate to observed galaxy-halo connections and the effects of mergers and feedback.

Contribution

It provides a detailed kinematic and photometric decomposition of simulated galaxies, linking their angular momentum properties to environmental and evolutionary factors.

Findings

Galaxies follow observed $j_*$--$M_*$ sequences after mergers.

Retention factors for disks are close to unity, for bulges are smaller.

Angular momentum growth is quasi-homologous between galaxies and halos.

Abstract

We investigate the angular momentum evolution of four disk galaxies residing in Milky Way-sized halos formed in cosmological zoom-in simulations with various sub-grid physics and merging histories. We decompose these galaxies kinematically and photometrically, into their disk and bulge components. The simulated galaxies and their components lie on the observed sequences in the $j_*$--$M_*$ diagram relating the specific angular momentum and mass of the stellar component. We find that galaxies in low-density environments follow the relation $j_* \propto M_*^{\alpha}$ past major mergers, with $\alpha \sim 0.6$ in the case of strong feedback, when bulge-to-disk ratios are relatively constant, and $\alpha \sim 1.4$ in the other cases, when secular processes operate on shorter timescales. We compute the retention factors (i.e. the ratio of the specific angular momenta of stars and dark matter) for both disks and bulges and show that they vary relatively slowly after averaging over numerous but brief fluctuations. For disks, the retention factors are usually close to unity, while for bulges, they are a few times smaller. Our simulations therefore indicate that galaxies and their halos grow in a quasi-homologous way.