The MaGICC Baryon Cycle: The Enrichment History of Simulated Disc
  Galaxies
  Using cosmological galaxy formation simulations from the MaGICC project,
spanning more than three magnitudes in stellar mass (~10^7-3x10^{10} Msun), we
trace the baryonic cycle of infalling gas from the virial radius through to its
participation in the star formation process. An emphasis is placed upon the
temporal history of chemical enrichment during its passage through the corona
and CGM. We derive the distributions of time between gas crossing the virial
radius and being accreted to the star forming region (which allows mixing
within the corona), as well as the time between gas being accreted to the star
forming region and then forming stars (which allows mixing within the disc).
Significant numbers of stars are formed from gas that cycles back through the
hot halo after first accreting to the star forming region. Gas entering high
mass galaxies is pre-enriched in low mass proto-galaxies prior to entering the
virial radius of the central progenitor, with only small amounts of primordial
gas accreted, even at high redshift (z~5). After entering the virial radius,
significant further enrichment occurs prior to the accretion of the gas to the
star forming region, with gas that is feeding the star forming region
surpassing 0.1Z by z=0. Mixing with halo gas, itself enriched via galactic
fountains, is thus crucial in determining the metallicity at which gas is
accreted to the disc. The lowest mass simulation (Mvir~2x10^{10}Msun, with
M*~10^7Msun), by contrast, accretes primordial gas through the virial radius
and onto the disc at all times. Much like the classical analytical solutions to
the `G-dwarf problem', overproduction of low-metallicity stars is ameliorated
by the inefficiency of star formation. Finally, gas outflow/metal removal rates
from star forming regions as a function of galactic mass are presented.
