B1359+154: A six-image lens produced by A z similar or equal to 1 compact group of galaxies

Hubble Space Telescope (HET) V- and I-band observations show that the gravi tational lens B1359 + 154 consists of six images of a single z(s) = 3.235 r adio source and its star-forming host galaxy, produced by a compact group o f galaxies at z(l) similar or equal to 1. Very Long Baseline Array (VLBA) o bservations at 1.7 GHz. strongly support this conclusion, showing six compa ct cores with similar low-frequency radio spectra. B1359+154 is the first e xample of galaxy-scale gravitational lensing in which more than four images are observed of the same background source. The configuration is due to th e unique lensing mass distribution: three primary lens galaxies lying on th e vertices of a triangle separated by 0 " .7 similar or equal to 4 h(-1) kp c, inside the 1 " .7 diameter Einstein ring defined by the radio images. Th e gravitational potential has additional extrema within this triangle, crea ting a pair of central images that supplement the "standard" four-image geo metry of the outer components. Simple mass models, consisting of three lens galaxies constrained by HST and VLBA astrometry, naturally reproduce the o bserved image positions but must be finely tuned to fit the flux densities.