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; these 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 expression for the Comptonization. We begin by assuming the intracluster (IC) gas to be isothermal at the emission-weighted average temperature determined by a combined analysis of the ASCA and Ginga X-ray satellite observations. The results of ROSAT /PSPC observations are used to determine an isothermal model for the S-Z surface brightness. Fitting to this model, we determine the peak Comptonization to be y 0 = 3.73 + 0.47 −0.61 × 10 −4 . The uncertainty includes contributions due to statistical uncertainty in the detection, instrumental baseline, calibration, and density model. Combining the X-ray and S-Z measurements, we determine the Hubble constant to be H 0 ( q 0 =½)=60 + 40 −23 km s -1 Mpc -1 , where the uncertainty is dominated by the systematic difference in the ASCA - and Ginga -determined IC gas temperatures. ASCA observations 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 structure. Using the ASCA -determined thermal structure and keeping the emission-weighted average temperature the same as in the isothermal case, we find H 0 ( q 0 = ½) = 78 + 54 −28 km s -1 Mpc -1 . Including additional uncertainties due to cluster asphericity, peculiar velocity, IC gas clumping, and astrophysical confusion, we find H 0 ( q 0 =½)=78 + 60 −40 km s -1 Mpc -1 .