Suppression of Star Formation in Low-Mass Galaxies Caused by the
  Reionization of their Local Neighborhood
  Photoheating associated with reionization suppressed star formation in
low-mass galaxies. Reionization was inhomogeneous, however, affecting different
regions at different times. To establish the causal connection between
reionization and suppression, we must take this local variation into account.
We analyze the results of CoDa (`Cosmic Dawn') I, the first fully-coupled
radiation-hydrodynamical simulation of reionization and galaxy formation in the
Local Universe, in a volume large enough to model reionization globally but
with enough resolving power to follow all atomic-cooling galactic halos in that
volume. For every halo identified at a given time, we find the redshift at
which the surrounding IGM reionized, along with its instantaneous star
formation rate (`SFR') and baryonic gas-to-dark matter ratio
($M_\text{gas}/M_\text{DM}$). The average SFR per halo with $M < 10^9 \text{
M}_\odot$ was steady in regions not yet reionized, but declined sharply
following local reionization. For $M > 10^{10} \text{ M}_\odot$, this SFR
continued through local reionization, increasing with time, instead. For $10^9
< M < 10^{10} \text{ M}_\odot$, the SFR generally increased modestly through
reionization, followed by a modest decline. In general, halo SFRs were higher
for regions that reionized earlier. A similar pattern was found for
$M_\text{gas}/M_\text{DM}$, which declined sharply following local reionization
for $M < 10^9 \text{ M}_\odot$. Local reionization time correlates with local
matter overdensity, which determines the local rates of structure formation and
ionizing photon consumption. The earliest patches to develop structure and
reionize ultimately produced more stars than they needed to finish and maintain
their own reionization, exporting their `surplus' starlight to help reionize
regions that developed structure later.
