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 _sstarf (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 _sstarf 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 _sstarf over a much wider halo mass range than with group or cluster catalogs. Second, we derive f _sstarf 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 _sstarf using abundance matching methods and show that the statistical uncertainty on f _sstarf (~10%) is dwarfed by systematic uncertainties associated with stellar mass measurements (~45% excluding initial mass function, IMF, uncertainties). Assuming a Chabrier IMF, we find 0.012 <= f _sstarf <= 0.025 at M _500c = 10¹³ M _⊙ and 0.0057 <= f _sstarf <= 0.015 at M _500c = 10¹⁴ M _⊙. These values are significantly lower than previously published estimates. We investigate the cause of this difference and find that previous work has overestimated f _sstarf 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 _sstarf is always significantly lower than f _gas for halos above 10¹³ M _⊙. Combining our results with recently published gas mass fractions, we find a shortfall in f _sstarf + f _gas at R _500c compared to the cosmic mean. This shortfall varies with halo mass and becomes larger toward lower halo masses.