Using data from the COSMOS survey, we perform the first joint analysis of galaxy-galaxy weak lensing, galaxy spatial clustering, and galaxy number densities. Carefully accounting for sample variance and for scatter between stellar and halo mass, we model all three observables simultaneously using a novel and self-consistent theoretical framework. Our results provide strong constraints on the shape and redshift evolution of the stellar-to-halo mass relation (SHMR) from z = 0.2 to z = 1. At low stellar mass, we find that halo mass scales as M h ∝ M 0.46 * and that this scaling does not evolve significantly with redshift from z = 0.2 to z = 1. The slope of the SHMR rises sharply at M * > 5 × 10 10 M ⊙ and as a consequence, the stellar mass of a central galaxy becomes a poor tracer of its parent halo mass. We show that the dark-to-stellar ratio, M h /M * , varies from low to high masses, reaching a minimum of M h /M * ∼ 27 at M * = 4.5 × 10 10 M ⊙ and M h = 1.2 × 10 12 M ⊙ . This minimum is important for models of galaxy formation because it marks the mass at which the accumulated stellar growth of the central galaxy has been the most efficient. We describe the SHMR at this minimum in terms of the "pivot stellar mass", M piv * , the "pivot halo mass", M piv h , and the "pivot ratio", (M h /M * ) piv . Thanks to a homogeneous analysis of a single data set spanning a large redshift range, we report the first detection of mass downsizing trends for both M piv h and M piv * . The pivot stellar mass decreases from M piv * = 5.75±0.13×10 10 M ⊙ at z = 0.88 to M piv * = 3.55±0.17×10 10 M ⊙ at z = 0.37. Intriguingly, however, the corresponding evolution of M piv h leaves the pivot ratio constant with redshift at (M h /M * ) piv ∼ 27. We use simple arguments to show how this result raises the possibility that star formation quenching may ultimately depend on M h /M * and not simply M h , as is commonly assumed. We show that simple models with such a dependence naturally lead to downsizing in the sites of star formation. Finally, we discuss the implications of our results in the context of popular quenching models, including disk instabilities and AGN feedback.

Studies of the diffuse x-ray-emitting gas in galaxy clusters have provided powerful constraints on cosmological parameters and insights into plasma astrophysics. However, measurements of the faint cluster outskirts have become possible only recently. Using data from the Suzaku x-ray telescope, we determined an accurate, spatially resolved census of the gas, metals, and dark matter out to the edge of the Perseus Cluster. Contrary to previous results, our measurements of the cluster baryon fraction are consistent with the expected universal value at half of the virial radius. The apparent baryon fraction exceeds the cosmic mean at larger radii, suggesting a clumpy distribution of the gas, which is important for understanding the ongoing growth of clusters from the surrounding cosmic web.

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 ⋆ as a function of halo mass M 500c from z = 0.2 to z = 1 using two complementary methods. First, we derive f ⋆ 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 ⋆ over a much wider halo mass range than with group or cluster catalogs. Second, we derive f ⋆ 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 ⋆ using abundance matching methods and we show that the statistical uncertainty on f ⋆ (∼ 10%) is dwarfed by systematic uncertainties associated with stellar mass measurements (∼ 45% excluding IMF uncertainties). Assuming a Chabrier IMF, we find 0.012 ≤ f ⋆ ≤ 0.025 at M 500c = 10 13 M ⊙ and 0.0057 ≤ f ⋆ ≤ 0.015 at M 500c = 10 14 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 ⋆ 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 ⋆ is always significantly lower than f gas for halos above 10 13 M ⊙ . Combining our results with recently published gas mas fractions, we find a shortfall in f ⋆ +f gas at R 500c compared to the cosmic mean. This shortfall varies with halo mass and becomes larger towards lower halos masses.

Measurements of X-ray scaling laws are critical for improving cosmological constraints derived with the halo mass function and for understanding the physical processes that govern the heating and cooling of the intracluster medium. In this paper, we use a sample of 206 X-ray selected galaxy groups to investigate the scaling relation between X-ray luminosity (L X ) and halo mass (M 200 ) where M 200 is derived via stacked weak gravitational lensing. This work draws upon a broad array of multiwavelength COSMOS observations including 1.64 degrees 2 of contiguous imaging with the Advanced Camera for Surveys (ACS) to a limiting magnitude of I F814W = 26.5 and deep XMM-Newton/Chandra imaging to a limiting flux of 1.0 × 10 −15 erg cm −2 s −1 in the 0.5-2 keV band. The combined depth of these two data-sets allows us to probe the lensing signals of X-ray detected structures at both higher redshifts and lower masses than previously explored. Weak lensing profiles and halo masses are derived for nine sub-samples, narrowly binned in luminosity and redshift. The COSMOS data alone are well fit by a power law, M 200 ∝ (L X ) α , with a slope of α = 0.66 ± 0.14. These results significantly extend the dynamic range for which the halo masses of X-ray selected structures have been measured with weak gravitational lensing. As a result, tight constraints are obtained for the slope of the M − L X relation. The combination of our group data with previously published cluster data demonstrates that the M − L X relation is well described by a single power law, α = 0.64 ± 0.03, over two decades in mass, M 200 ∼ 10 13.5 -10 15.5 h −1 72 M . These results are inconsistent at the 3.7σ level with the self-similar prediction of α = 0.75. We examine the redshift dependence of the M − L X relation and find little evidence for evolution beyond the rate predicted by self-similarity from z ∼ 0.25 to z ∼ 0.8.

Understanding the mechanisms that lead dense environments to host galaxies with redder colors, more spheroidal morphologies, and lower star formation rates than field populations remains an important problem. As most candidate processes ultimately depend on host halo mass, accurate characterizations of the local environment, ideally tied to halo mass estimates and spanning a range in halo mass and redshift are needed. In this work, we present and test a rigorous, probabalistic method for assigning galaxies to groups based on precise photometric redshifts and X-ray selected groups drawn from the COSMOS field. The groups have masses in the range 10 13 M 200c /M 10 14 and span redshifts 0 < z < 1. We characterize our selection algorithm via tests on spectroscopic subsamples, including new data obtained at the VLT, and by applying our method to detailed mock catalogs. We find that our group member galaxy sample has a purity of 84% and completeness of 92% within 0.5R 200c . We measure the impact of uncertainties in redshifts and group centering on the quality of the member selection with simulations based on current data as well as future imaging and spectroscopic surveys. As a first application of our new group member catalog which will be made publicly available, we show that member galaxies exhibit a higher quenched fraction compared to the field at fixed stellar mass out to z ∼ 1, indicating a significant relationship between star formation and environment at group scales. We also address the suggestion that dusty star forming galaxies in such groups may impact the high-power spectrum of the cosmic microwave background and find that such a population cannot explain the low power seen in recent SZ measurements.