The z=2.72 galaxy cB58: a gravitational fold arc lensed by the cluster MS 1512+36

Using HST WFPC2 V- and R-band data of the z = 0.37 cluster MS 1512 + 36, we show that the z = 2.72 'protogalaxy' cB58 is not extraordinarily luminous intrinsically but lensed into a gravitational fold are by the cluster. The are has a surface-brightness-weighted axis ratio of 1:7, is marginally resolved in width and about 3 arcsec long. Its counterimage was identified and found to be very compact (r(1/2) = 2.4-4.0 h(50)(-1) kpc in a q(0) = 0.05 cosmology). In addition, we found three further multiple image systems, one with five and two with three images each. The positions of the multiple images can be explained by modelling the light deflection caused by the duster and the cD galaxy with elliptical isothermal potentials. The major axis of the cluster potential approximately agrees with that of the cD light and that of the X-ray isophotes. As the multiple images are within approximate to 8 arcsec around the cD galaxy, a cluster core radius-cluster velocity dispersion degeneracy arises. Interpreting the observations conservatively, the cluster velocity dispersion and the core radius are limited to 540-670 km s(-1) and 5-11 arcsec, respectively, and the brightness of the unlensed counterimage of cB58 is about 23.9 +/- 0.3 (R band), corresponding to a magnification-and extinction-corrected rest frame B-band absolute magnitude of -24.75 +/- 0.7 mag. The redshifts of the sources of the remaining three multiple image systems are predicted to be similar to that of cB58, while a strict upper limit of 4 is set as they are visible in B-band ground-based data. That part of the source of cB58 that is mapped into the are is reconstructed and its magnification is found to be mu(arc)greater than or similar to 50. This large magnification explains at least some of the untypical spectroscopic properties of cB58, e.g. that the star formation rate seems to be high and uniform and to take place in a large area.