CLASS B1555+375: A New Four-Image Gravitational Lens System

We have discovered a new gravitational lens in the Cosmic Lens All-Sky Survey. The lens B1555+375 is a four-image system with a maximum separation of 0.”42. VLA and MERLIN radio observations show these images in a characteristic quadruple-lens configuration. Optical imaging with the Keck II Telescope at R band shows a faint extended object. We estimate the combined emission from the lens and background source to be R = 25 mag. Observations at H band with the William Herschel Telescope also detected this extended object. The combined lens and background source magnitude was measured to be H = 19 mag. Presently, redshifts for both the lensing galaxy and background source are undetermined. We conclude that one or more of the following are true: the lens galaxy is sub-L*, it has z > 0.5, or it is highly reddened. The observed MERLIN component positions and flux densities can be well described by a lens model based on a singular isothermal ellipsoid mass distribution. Our best-fit lens model has a reduced χ^2 of 2.6. High-resolution VLA radio observations should help to constrain the lens model further.