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 .