We have observed the Sunyaev–Zel ’ dovich effect in a sample of five moderate-redshift clusters with the Ryle Telescope , and used them in conjunction with X-ray imaging and spectral data from ROSAT and ASCA to measure the Hubble constant .
This sample was chosen with a strict X-ray flux limit using both the BCS and NORAS cluster catalogues to be well above the surface-brightness limit of the ROSAT All-Sky Survey , and hence to be unbiased with respect to the orientation of the cluster .
This controls the major potential systematic effect in the SZ/X-ray method of measureing H _ { 0 } .
Taking the weighted geometric mean of the results and including the main sources of random error , namely the noise in the SZ measurement , the uncertainty in the X-ray temperatures and the unknown ellipticity of the clusters , we find H _ { 0 } = 59 ^ { +8 } _ { -7 } km s ^ { -1 } Mpc ^ { -1 } assuming a standard CDM model with \Omega _ { M } = 1.0 , \Omega _ { \Lambda } = 0.0 , or H _ { 0 } = 65 ^ { +8 } _ { -7 } km s ^ { -1 } Mpc ^ { -1 } if \Omega _ { M } = 0.3 , \Omega _ { \Lambda } = 0.7 .