ELUCID IV: Galaxy Quenching and its Relation to Halo Mass, Environment,
  and Assembly Bias
  We examine the quenched fraction of central and satellite galaxies as a
function of galaxy stellar mass, halo mass, and the matter density of their
large scale environment. Matter densities are inferred from our ELUCID
simulation, a constrained simulation of local Universe sampled by SDSS, while
halo masses and central/satellite classification are taken from the galaxy
group catalog of Yang et al. The quenched fraction for the total population
increases systematically with the three quantities. We find that the
`environmental quenching efficiency', which quantifies the quenched fraction as
function of halo mass, is independent of stellar mass. And this independence is
the origin of the stellar mass-independence of density-based quenching
efficiency, found in previous studies. Considering centrals and satellites
separately, we find that the two populations follow similar correlations of
quenching efficiency with halo mass and stellar mass, suggesting that they have
experienced similar quenching processes in their host halo. We demonstrate that
satellite quenching alone cannot account for the environmental quenching
efficiency of the total galaxy population and the difference between the two
populations found previously mainly arises from the fact that centrals and
satellites of the same stellar mass reside, on average, in halos of different
mass. After removing these halo-mass and stellar-mass effects, there remains a
weak, but significant, residual dependence on environmental density, which is
eliminated when halo assembly bias is taken into account. Our results therefore
indicate that halo mass is the prime environmental parameter that regulates the
quenching of both centrals and satellites.
