THE DARK-MATTER, GAS, AND GALAXY DISTRIBUTIONS IN ABELL-2218 - A WEAKGRAVITATIONAL LENSING AND X-RAY-ANALYSIS

We report on the detection of dark matter in the cluster Abell 2218 us ing the weak gravitational distortion of background galaxies. We find a highly significant, coherent detection of the distortion in the imag es of the background galaxies. We use Hubble Space Telescope (HST) ima ges from the Medium Deep Survey to calibrate the suppression in the ob served distortion due to atmospheric smearing. The inferred two-dimens ional mass distribution has a peak that is coincident with the optical and X-ray centroid. The qualitative distributions of the cluster ligh t, the X-ray emission, and the dark matter are similar and the project ed total mass, gas, and light surface densities are consistent with an r(-1) profile at a distance of r > 180 '' from the cluster cD galaxy. Using the weak lensing technique, we determine a lower bound for the total mass in A2218 of (3.9 +/- 0.7) x 10(14) h(-1) M(.) within a fidu cial aperture of radius 0.4 h(-1) Mpc. The associated cluster mass-to- light ratio is (440 +/- 80) h M(.)/L(.B). The mass estimated by the we ak lensing method is consistent with that inferred from the X-ray data under the assumption of hydrostatic equilibrium, and we derive an upp er bound for the gas-to-total mass ratio at 400 h(-1) kpc of M(gas)/M( tot) = (0.04 +/- 0.02) h(-3/2). The lensing estimates assumes that a c ontrol annulus extending from 0.4-0.7 h(-1) Mpc is empty. Correcting f or the mean surface density matter in the control annulus as inferred from the X-ray data increases the lensing mass estimate by similar or equal to 20%, and the lensing and X-ray mass estimates are still consi stent within the experimental uncertainties.