The integrated stellar content of dark matter halos
  Measurements of the total amount of stars locked up in galaxies as a function
of host halo mass contain key clues about the efficiency of processes that
regulate star formation. We derive the total stellar mass fraction f_star as a
function of halo mass M500c from z=0.2 to z=1 using two complementary methods.
First, we derive f_star using a statistical Halo Occupation Distribution model
jointly constrained by data from lensing, clustering, and the stellar mass
function. This method enables us to probe f_star over a much wider halo mass
range than with group or cluster catalogs. Second, we derive f_star at group
scales using a COSMOS X-ray group catalog and we show that the two methods
agree to within 30%. We quantify the systematic uncertainty on f_star using
abundance matching methods and we show that the statistical uncertainty on
f_star (~10%) is dwarfed by systematic uncertainties associated with stellar
mass measurements (~45% excluding IMF uncertainties). Assuming a Chabrier IMF,
we find 0.012<f_star<0.025 at M500c=10^13 Msun and 0.0057<f_star<0.015 at
M500c=10^14 Msun. These values are significantly lower than previously
published estimates. We investigate the cause of this difference and find that
previous work has overestimated f_star due to a combination of inaccurate
stellar mass estimators and/or because they have assumed that all galaxies in
groups are early type galaxies with a constant mass-to-light ratio. Contrary to
previous claims, our results suggest that the mean value of f_star is always
significantly lower than f_gas for halos above 10^13 Msun. Combining our
results with recently published gas mas fractions, we find a shortfall in
f_star+f_gas at R500c compared to the cosmic mean. This shortfall varies with
halo mass and becomes larger towards lower halos masses.
