A Photometric and Weak Lensing Analysis of the z=0.42 Supercluster MS0302+17

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We perform a weak lensing and photometric study of the z=0.42 supercluster MS0302+17 using deep I and V band images taken with the UH8K CCD mosaic camera at the CFHT. We use archival ROSAT HRI data to estimate fluxes, gas masses and, in one case, the binding mass of the three major clusters. We then use our CCD data to determine the optical richness and luminosities of the clusters and to map out the spatial distribution of the early type galaxies in the supercluster and in other foreground and background structures. We measure the gravitational shear from a sample of ≃ 30,000 faint background galaxies in the range 22 < mᵢ < 26 and find this correlates strongly with that predicted from the early type galaxies if they trace the mass with M/LB ≃ 250 h. We make 2-dimensional reconstructions of the mass surface density. These recover all of the major concentrations of galaxies and indicate that most of the supercluster mass, like the early type galaxies, is concentrated in the three X-ray clusters, and we obtain mean mass-to-light ratios for the clusters of M/LB ≃ 260 h. Cross-correlation of the measured mass surface density with that predicted from the early type galaxy distribution shows a strong peak at zero lag (significant at the ≃ 9-sigma level), and that at separations 200 h⁻¹kpc the early galaxies trace the mass very accurately. This conclusion is supported by cross-correlation in Fourier space; we see little evidence for any variation of M/L or `bias' with scale, and from the longest wavelength modes with λ= 1.5-6 h⁻¹Mpc we find M/L ≃ (280 ± 40)h, quite similar to that obtained for the cluster centers. We discuss the implication of these results for the cosmological density parameter.