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) (excluding stars in the intracluster light) as a function of halo mass M-500c 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 show that the two methods agree to within 30%. We quantify the systematic uncertainty on f(star) using abundance matching methods and show that the statistical uncertainty on f(star) (similar to 10%) is dwarfed by systematic uncertainties associated with stellar mass measurements (similar to 45% excluding initial mass function, IMF, uncertainties). Assuming a Chabrier IMF, we find 0.012 <= f(star) <= 0.025 at M-500c = 10(13) M-circle dot and 0.0057 <= f(star) <= 0.015 at M-500c = 10(14) M-circle dot. 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) owing 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 1013M(circle dot). Combining our results with recently published gas mass fractions, we find a shortfall in f(star) + f(gas) at R-500c compared to the cosmic mean. This shortfall varies with halo mass and becomes larger toward lower halo masses.