Modeling Galactic Conformity with the Color-Halo Age Relation in the
  Illustris Simulation
  Comparisons between observational surveys and galaxy formation models find
that the mass of dark matter haloes can largely explain galaxies' stellar mass.
However, it remains uncertain whether additional environmental variables,
generally referred to as assembly bias, are necessary to explain other galaxy
properties. We use the Illustris Simulation to investigate the role of assembly
bias in producing galactic conformity by considering 18,000 galaxies with
$M_{stellar}$ > $2 \times 10^9$ $M_{\odot}$. We find a significant signal of
galactic conformity: out to distances of about 10 Mpc, the mean red fraction of
galaxies around redder galaxies is higher than around bluer galaxies at fixed
stellar mass. Dark matter haloes exhibit an analogous conformity signal, in
which the fraction of haloes formed at earlier times (old haloes) is higher
around old haloes than around younger ones at fixed halo mass. A plausible
interpretation of galactic conformity can be given as a combination of the halo
conformity signal with the galaxy color-halo age relation: at fixed stellar
mass, particularly toward the low-mass end, Illustris' galaxy colors correlate
with halo age, with the reddest galaxies (often satellites) being
preferentially found in the oldest haloes. In fact, we can explain the galactic
conformity effect with a simple semi-empirical model, by assigning stellar mass
based on halo mass (abundance matching) and by assigning galaxy color based on
halo age (age matching). We investigate other interpretations for the galactic
conformity, particularly its dependence on the isolation criterion and on the
central-satellite information. Regarding comparison to observations, we
conclude that the adopted selection/isolation criteria, projection effects, and
stacking techniques can have a significant impact on the measured amplitude of
the conformity signal.
