MEASUREMENT OF THE HUBBLE CONSTANT FROM X-RAY AND 2.1 MILLIMETER OBSERVATIONS OF ABELL-2163

We report 2.1 mm observations of the Sunyaev-Zeldovich (S-Z) effect; t hese observations confirm our previous detection of a decrement in the cosmic microwave background intensity toward the cluster Abell 2163. The S-Z data are analyzed using the relativistically correct expressio n for the Comptonization. We begin by assuming the intracluster (IC) g as to be isothermal at the emission-weighted average temperature deter mined by a combined analysis of the ASCA and Ginga X-ray satellite obs ervations. The results of ROSAT/PSPC observations are used to determin e an isothermal model for the S-Z surface brightness. Fitting to this model, we determine the peak Comptonization to be y(0) = 3.73(-0.61)(0.47) x 10(-4). The uncertainty includes contributions due to statisti cal uncertainty in the detection, instrumental baseline, calibration, and density model. Combining the X-ray and S-Z measurements, we determ ine the Hubble constant to be H-0(q(0) = 1/2) = 60(-23)(+40) km s(-1) Mpc(-1), where the uncertainty is dominated by the systematic differen ce in the ASCA- and Ginga-determined IC gas temperatures. ASCA observa tions suggest the presence of a significant thermal gradient in the IC gas. We determine H-0 as a function of the assumed IC gas thermal str ucture. Using the ASCA-determined thermal structure and keeping the em ission-weighted average temperature the same as in the isothermal case , we find H-0(q(0) = 1/2) = 78(-28)(+54) km s(-1) Mpc(-1). Including a dditional uncertainties due to cluster asphericity, peculiar velocity, IC gas clumping, and astrophysical confusion, we find H-0(q(0) = 1/2) = 78(-40)(+60) km s(-1) Mpc(-1).