Strong bimodality in the host halo mass of central galaxies from
  galaxy-galaxy lensing
  We use galaxy-galaxy lensing to study the dark matter halos surrounding a
sample of Locally Brightest Galaxies (LBGs) selected from the Sloan Digital Sky
Survey. We measure mean halo mass as a function of the stellar mass and colour
of the central galaxy. Mock catalogues constructed from semi-analytic galaxy
formation simulations demonstrate that most LBGs are the central objects of
their halos, greatly reducing interpretation uncertainties due to satellite
contributions to the lensing signal. Over the full stellar mass range, $10.3 <
\log [M_*/M_\odot] < 11.6$, we find that passive central galaxies have halos
that are at least twice as massive as those of star-forming objects of the same
stellar mass. The significance of this effect exceeds $3\sigma$ for $\log
[M_*/M_\odot] > 10.7$. Tests using the mock catalogues and on the data
themselves clarify the effects of LBG selection and show that it cannot
artificially induce a systematic dependence of halo mass on LBG colour. The
bimodality in halo mass at fixed stellar mass is reproduced by the
astrophysical model underlying our mock catalogue, but the sign of the effect
is inconsistent with recent, nearly parameter-free age-matching models. The
sign and magnitude of the effect can, however, be reproduced by halo occupation
distribution models with a simple (few-parameter) prescription for
type-dependence.
