A MEASUREMENT OF THE HUBBLE CONSTANT FROM THE X-RAY-PROPERTIES AND THE SUNYAEV-ZELDOVICH EFFECT OF ABELL 2218

The distance of the cluster of galaxies Abell 2218, and hence the valu e of the Hubble constant, has been measured by comparing the X-ray pro perties and the Sunyaev-Zel'dovich effect of the cluster. The result f or the Hubble constant is H0 = 65 +/- 25 km s-1 Mpc-1, where the error includes the random and systematic errors, combined in quadrature, an d the largest contributions to the overall error come from systematic errors in the Sunyaev-Zel'dovich effect data and in the X-ray spectrum of the cluster. The X-ray and Sunyaev-Zel'dovich effect data are cons istent with the same model for the intergalactic medium in Abell 2218 (an isothermal beta-model with beta almost-equal-to 0.65 and cluster c ore radius almost-equal-to 1.'0). A previous report of a smaller Hubbl e constant, found by applying the same method on the same cluster, is shown to be based on a model for the cluster gas that is inconsistent with the Einstein IPC data. The present result for the Hubble constant is consistent with the value found earlier for the cluster Abell 665: by combining the results of the method for both clusters we conclude that the value of the Hubble constant is H0 = 55 +/- 17 km s-1 Mpc-1.