The mass distribution of the cluster 0957+561 from gravitational lensing

Multiply gravitationally lensed objects with known time delays can lead to direct determinations of H-0 independent of the distance ladder if the mass distribution of the lens is known. Currently, the double QSO 0957+561 is the only lensed object with a precisely known time delay, The largest remaining source of systematic error in the H-0 determination results from uncertainty in the mass distribution of the lens which is comprised of a massive galaxy (G1) and the cluster in which it resides, We have obtained V-band CCD images from CFHT in order to measure the mass distribution in the cluster from its gravitional distorting effect on the appearance of background galaxies. We use this data to construct a two-dimensional mass map of the field. A mass peak is detected at the 4.5 sigma level, offset from, but consistent with, the position of G1. Simple tests reveal no significant substructure and the mass distribution is consistent with a spherical cluster. The peak in the number density map of bright galaxies is offset from G1 similarly to the mass peak. We constructed an azimuthally averaged mass profile centered on G1 out to 2' (400h(-1) kpc). It is consistent with an isothermal mass distribution with a small core (r(c) approximate to 5'' = 17h(-1) kpc). The inferred mass within 1 Mpc is consistent with the dynamical mass estimate but 2 sigma higher than the upper limits from a ROSAT X-ray study. We discuss implications for H-0 in a future paper. (C) 1997 American Astronomical Society.