Gas accretion and galactic fountain flows in the Auriga cosmological
  simulations: angular momentum and metal re-distribution
  Using a set of 15 high-resolution magnetohydrodynamic cosmological
simulations of Milky Way formation, we investigate the origin of the baryonic
material found in stars at redshift zero. We find that roughly half of this
material originates from subhalo/satellite systems and half is smoothly
accreted from the Inter-Galactic Medium (IGM). About $90 \%$ of all material
has been ejected and re-accreted in galactic winds at least once. The vast
majority of smoothly accreted gas enters into a galactic fountain that extends
to a median galactocentric distance of $\sim 20$ kpc with a median recycling
timescale of $\sim 500$ Myr. We demonstrate that, in most cases, galactic
fountains acquire angular momentum via mixing of low-angular momentum,
wind-recycled gas with high-angular momentum gas in the Circum-Galactic Medium
(CGM). Prograde mergers boost this activity by helping to align the disc and
CGM rotation axes, whereas retrograde mergers cause the fountain to lose
angular momentum. Fountain flows that promote angular momentum growth are
conducive to smooth evolution on tracks quasi-parallel to the disc sequence of
the stellar mass-specific angular momentum plane, whereas retrograde minor
mergers, major mergers and bar-driven secular evolution move galaxies towards
the bulge-sequence. Finally, we demonstrate that fountain flows act to flatten
and narrow the radial metallicity gradient and metallicity dispersion of disc
stars, respectively. Thus, the evolution of galactic fountains depends strongly
on the cosmological merger history and is crucial for the chemo-dynamical
evolution of Milky Way-sized disc galaxies.
