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