DARK-MATTER IN MS-1224 FROM DISTORTION OF BACKGROUND GALAXIES

We explore the dark matter distribution in MS 1224.7+2007 using the gravitational distortion of the images of faint background galaxies. Projected mass image reconstruction reveals a highly significant concentration coincident with the X-ray and optical location. The concentration is seen repeatably in reconstructions from independent subsamples, and the azimuthally averaged tangential shear pattern is also clearly seen in the data. The projected mass within a 2'.76 radius aperture is similar or equal to 3.5 x 10(14) h(-1) M.. This is similar or equal to 3 times larger than that predicted if mass traces light with M/L = 275 as derived from virial analysis. It is very hard to attribute the discrepancy to a statistical fluctuation, and a further indication of a significant difference between the mass and the light comes from a second mass concentration, which is again seen in independent subsamples but which is not seen at all in the cluster light. We find a mass per galaxy visible to I = 22 of similar or equal to 8 x 10(12) h(-1) M., which, if representative of the universe, implies a density parameter Omega similar or equal to 2. We find a null detection of any net shear from large-scale structure with a precision of 0.9% per component. This is much smaller than the possible detection in a recent comparable study, and the precision here is at about the level of the rms shear fluctuations in realistic models.