1. Relating Mass and Light in the COSMOS Field J.E. Taylor, R.J. Massey ( California Institute of Technology), J. Rhodes ( Jet Propulsion Laboratory) & the COSMOS Team Introduction The COSMOS survey (Scoville et al. 2006) provides an unprecedented combination of sensitive gravitational lensing maps, high-resolution X- ray imaging, and multi-band optical observations over a 2-square degree field. This unique data set can be used to trace the growth of structure on a wide range of spatial scales, and from the local universe to high redshift. In particular, independent lensing and X-ray information can be used to determine the mass, age, and thermodynamic state for a large sample of groups and clusters. It also serves as a testing ground for new methods of reconstructing the full 3-dimensional matter distribution, and characterizing the dependence of galaxy properties on local environment. Lensing Peaks vs. X-ray Clusters Galaxy clusters are strong X-ray sources, visible out to moderate redshift, and should also appear as peaks in the weak lensing mass maps. The abundance of clusters at high redshift is a sensitive test of cosmology which many future surveys hope to exploit. We can test how X-ray emission and cluster mass are related in the COSMOS field, by comparing objects in the X-ray group and cluster catalogue (Finoguenov et al. 2006) with a map of weak lensing convergence. Even in projection, roughly 60% of the lensing peaks correspond to massive systems in the X-ray catalogue, confirming recent predictions (Hamana et al. 2004). 3-D Analysis The COSMOS data set will eventually include spectroscopic redshifts for 50000 objects. We can already use the multi-band photometry to estimate approximate redshifts for 1 Million or more galaxies. While less accurate, these photometric redshifts are well suited to lensing studies, where statistical averages over hundreds of galaxies are required to obtain reasonable signal-to-noise. We illustrate an initial use of information in the third dimension below. Galaxy Properties vs. Environment The weak lensing measurement of projected mass (convergence) opens up a whole new range of parameter space for studies of environmental dependence in galaxy properties. As an initial indication, we can bin galaxies by the local convergence in a medium-redshift lensing mass (sensitive to mass between z ~ 0.2 and 0.9), and see how galaxy properties change in denser regions. References See COSMOS 2006 Ap.J. Special Issue papers, especially: Scoville et al. 2006 (survey); Rhodes et al. 2006, Massey et al 2006, Leauthaud et al 2006 (weak lensing); Capak et al. 2006; Mobasher et al. 2006, Tasca et al. 2006 (photometry and redshifts); Finoguenov et al. 2006 (X-ray catalogue) Also: Hamana, Takada & Yoshida 2004 MNRAS 350, 893 Taylor et al. 2006 in preparation Mean tangential shear vs. angular diameter distance, along lines of sight around peaks in the 2-D map. Increasing positive shear beyond a particular distance indicates the presence of a cluster at that distance Convergence Bin Morphological T log(Stellar Mass) Correspondence between lensing mass map (background image) and X-ray groups and clusters (circles, scaled to R 500,with redshift).