  Recent studies suggest that the quenching properties of galaxies are
correlated over several mega-parsecs. The large-scale "galactic conformity"
phenomenon around central galaxies has been regarded as a potential signature
of "galaxy assembly bias" or "pre-heating", both of which interpret conformity
as a result of direct environmental effects acting on galaxy formation.
Building on the iHOD halo quenching framework developed in Zu & Mandelbaum
(2015, 2016), we discover that our fiducial halo mass quenching model, without
any galaxy assembly bias, can successfully explain the overall environmental
dependence and the conformity of galaxy colours in SDSS, as measured by the
mark correlation functions of galaxy colours and the red galaxy fractions
around isolated primaries, respectively. Our fiducial iHOD halo quenching mock
also correctly predicts the differences in the spatial clustering and
galaxy-galaxy lensing signals between the more vs. less red galaxy subsamples,
split by the red-sequence ridge-line at fixed stellar mass. Meanwhile, models
that tie galaxy colours fully or partially to halo assembly bias have
difficulties in matching all these observables simultaneously. Therefore, we
demonstrate that the observed environmental dependence of galaxy colours can be
naturally explained by the combination of 1) halo quenching and 2) the
variation of halo mass function with environment --- an indirect environmental
effect mediated by two separate physical processes.
