Halo Histories vs. Galaxy Properties at z=0, I: The Quenching of Star
  Formation
  We test whether halo age and galaxy age are correlated at fixed halo and
galaxy mass. The formation histories, and thus ages, of dark matter halos
correlate with their large-scale density $\rho$, an effect known as assembly
bias. We test whether this correlation extends to galaxies by measuring the
dependence of galaxy stellar age on $\rho$. To clarify the comparison between
theory and observation, and to remove the strong environmental effects on
satellites, we use galaxy group catalogs to identify central galaxies and
measure their quenched fraction, $f_Q$, as a function of large-scale
environment. Models that match halo age to central galaxy age predict a strong
positive correlation between $f_Q$ and $\rho$. However, we show that the
amplitude of this effect depends on the definition of halo age: assembly bias
is significantly reduced when removing the effects of splashback halos---those
halos that are central but have passed through a larger halo or experienced
strong tidal encounters. Defining age using halo mass at its peak value rather
than current mass removes these effects. In SDSS data, at M$_{\rm gal}\gtrsim
10^{10.0}$ M_sol/h$^2$, there is a $\sim 5\%$ increase in $f_Q$ from low to
high densities, which is in agreement with predictions of dark matter halos
using peak halo mass. At lower stellar mass there is little to no correlation
of $f_Q$ with $\rho$. For these galaxies, age-matching is inconsistent with the
data across the wide range the halo formation metrics that we tested. This
implies that halo formation history has a small but statistically significant
impact on quenching of star formation at high masses, while the quenching
process in low-mass central galaxies is uncorrelated with halo formation
history.
