The Hubble Constant from gravitational lens CLASS B0218+357 using the Advanced Camera for Surveys

We present deep optical observations of the gravitational lens system CLASS B0218+357 from which we derive an estimate for the Hubble Constant (H0) Extensive radio observations using the VLA, MERLIN, the VLBA and VLBI have reduced the degeneracies between H0 and the mass model parameters in this lens to one involving only the position of the radioquiet lensing galaxy with respect to the lensed images. B0218+357 has an image separation of only 334 mas, so optical observations have, up until now, been unable to resolve the lens galaxy from the bright lensed images. Using the new Advanced Camera for Surveys, installed on the Hubble Space Telescope in 2002, we have obtained deep optical images of the lens system and surrounding field. These observations have allowed us to determine the separation between the lens galaxy centre and the brightest image, and hence estimate H0. We find H0 = 73±8 km sMpc(68% confidence). This estimate is very similar to the local value from the Hubble Key Project and the value from WMAP, and is in agreement with the value deduced from radio observations of B0218+357.