Growing galaxies via superbubble-driven accretion flows
  We use a suite a cooling halo simulations to study a new mechanism for rapid
accretion of hot halo gas onto star-forming galaxies. Correlated supernovae
events create converging 'superbubbles' in the halo gas. Where these collide,
the density increases, driving cooling filaments of low metallicity gas that
feed the disc. At our current numerical resolution (20 pc) we are only able to
resolve the most dramatic events; these could be responsible for the build-up
of galaxy discs after the most massive gas-rich mergers have completed (z < 1).
As we increase the numerical resolution, we find that the filaments persist for
longer, driving continued late-time star formation. This suggests that
SNe-driven accretion could act as an efficient mechanism for extracting cold
gas from the hot halo, driving late-time star formation in disc galaxies. We
show that such filament feeding leads to a peak star formation rate (SFR) of
$\sim 3$ M$_{\rm sun}$ yr$^{-1}$, consistent with estimates for the Milky Way.
By contrast, direct cooling from the hot halo ('hot-mode' accretion, not
present in the simulations that show filament feeding) falls short of the
SNe-driven SFR by a factor of 3-4, and is sustained over a shorter time period.
The filaments we resolve extend to $\sim$ 50 kpc, reaching column densities of
$\sim 10^{18}$ cm$^{-2}$. We show that such structures can plausibly explain
the broad dispersion in Mg II absorption seen along sight lines to quasars. Our
results suggest a dual role for stellar feedback in galaxy formation,
suppressing hot-mode accretion while promoting cold-mode accretion along
filaments. This ultimately leads to more star formation, suggesting that the
positive feedback effect outweighs the negative. Finally, since the filamentary
gas has higher angular momentum than that coming from hot-mode accretion, we
show that this leads to the formation of substantially larger gas discs.
