On the Physical Origin of Galactic Conformity
  Correlations between the star formation rates (SFRs) of nearby galaxies
(so-called galactic conformity) have been observed for projected separations up
to 4 Mpc, an effect not predicted by current semi-analytic models. We
investigate correlations between the mass accretion rates (dMvir/dt) of nearby
halos as a potential physical origin for this effect. We find that pairs of
host halos "know about" each others' assembly histories even when their
present-day separation is greater than thirty times the virial radius of either
halo. These distances are far too large for direct interaction between the
halos to explain the correlation in their dMvir/dt. Instead, halo pairs at
these distances reside in the same large-scale tidal environment, which
regulates dMvir/dt for both halos. Larger halos are less affected by external
forces, which naturally gives rise to a mass dependence of the halo conformity
signal. SDSS measurements of galactic conformity exhibit a qualitatively
similar dependence on stellar mass, including how the signal varies with
distance. Based on the expectation that halo accretion and galaxy SFR are
correlated, we predict the scale-, mass- and redshift-dependence of large-scale
galactic conformity, finding that the signal should drop to undetectable levels
by z > 1. These predictions are testable with current surveys to z ~ 1;
confirmation would establish a strong correlation between dark matter halo
accretion rate and central galaxy SFR.
