Abstract
While satellite galaxies within groups/clusters are more likely to exhibit
quiescent star formation rates (SFR) than central (field) galaxies, an enhanced
quiescent likelihood extends to galaxies that are several virial radii beyond
groups/clusters. Using a galaxy group/cluster catalog from SDSS DR7, we show
that this enhancement has two causes. First, massive halos are clustered, so
neighboring halos that host their own satellite galaxies boost the observed
quiescent fraction out to large distances. Second, and more interestingly,
central galaxies exhibit a strongly enhanced quiescent fraction out to 2.5
virial radii (with some enhancement out to 5 virial radii) beyond
groups/clusters. We show that this enhancement for central galaxies can be
explained by 'ejected' satellite galaxies that orbit beyond their host halo's
virial radius. Using a cosmological N-body simulation, we examine the orbital
histories of ejected satellites, showing that they extend out to 2.5 virial
radii beyond their host halo and compose up to 40% of all central galaxies out
to this distance. After ejection, these satellites typically orbit for several
Gyr beyond the virial radius before falling back in, during which time they
(continue to) lose significant halo mass. We show that a model in which ejected
satellites experience the same SFR evolution and quenching as satellites within
a host halo can explain essentially all environmental dependence of galaxy
quenching beyond a host halo's virial radius. The strong halo mass stripping of
ejected satellites also reduces the average halo masses of galaxies near
massive groups/clusters, an effect that is potentially observable via weak
lensing. Overall, the SFRs/colors and stellar/halo masses of ejected satellites
present challenges for the standard halo model for galaxy occupation and
highlight the importance of environmental history in galaxy evolution.
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