  The relationship between galaxies and dark matter can be characterized by the
halo mass of the central galaxy and the fraction of galaxies that are
satellites. Here we present observational constraints from the SDSS on these
quantities as a function of r-band luminosity and stellar mass using
galaxy-galaxy weak lensing, with a total of 351,507 lenses. We use stellar
masses derived from spectroscopy and virial halo masses derived from weak
gravitational lensing to determine the efficiency with which baryons in the
halo of the central galaxy have been converted into stars. We find that an L*
galaxy with a stellar mass of 6x10^{10} M_{sun} is hosted by a halo with mass
of 1.4x10^{12} M_{sun}/h, independent of morphology, yielding baryon conversion
efficiencies of 17_{-5}^{+10} (early types) and 16_{-6}^{+15} (late types) per
cent at the 95 per cent CL (statistical, not including systematic uncertainty
due to assumption of a universal initial mass function, or IMF). We find that
for a given stellar mass, the halo mass is independent of morphology below
M_{stellar}=10^{11} M_{sun}, in contrast to typically a factor of two
difference in halo mass between ellipticals and spirals at a fixed luminosity.
This suggests that stellar mass is a good proxy for halo mass in this range and
should be used preferentially whenever a halo mass selected sample is needed.
For higher stellar masses, the conversion efficiency is a declining function of
stellar mass, and the differences in halo mass between early and late types
become larger, reflecting the fact that most group and cluster halos with
masses above 10^{13} M_{sun} host ellipticals at the center, while even the
brightest central spirals are hosted by halos of mass below 10^{13} M_{sun}.
(Abridged)
