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

We explore the dark matter distribution in MS 1224.7+2007 using the gr avitational distortion of the images of faint background galaxies. Pro jected mass image reconstruction reveals a highly significant concentr ation coincident with the X-ray and optical location. The concentratio n is seen repeatably in reconstructions from independent subsamples, a nd the azimuthally averaged tangential shear pattern is also clearly s een 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 = 27 5 as derived from virial analysis. It is very hard to attribute the di screpancy to a statistical fluctuation, and a further indication of a significant difference between the mass and the light comes from a sec ond 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., w hich, if representative of the universe, implies a density parameter O mega 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. Thi s is much smaller than the possible detection in a recent comparable s tudy, and the precision here is at about the level of the rms shear fl uctuations in realistic models.