2005MNRAS.357..124Y

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 H₀and the mass model parameters in this lens to one involving only the position of the radio-quiet 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 (ACS), 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 so estimate H₀.

We find an optical galaxy position, and hence an H₀value, that varies depending on our approach to the spiral arms in B0218+357. If the most prominent spiral arms are left unmasked, we find H₀= 70±5 km/s/Mpc (95 per cent confidence). If the spiral arms are masked out, we find H₀= 61±7 km/s/Mpc (95 per cent confidence).