* Astronomy

Title: CASSOWARY 20: a Wide Separation Einstein Cross Identified with the X-shooter Spectrograph
Authors: Max Pettini (1), Lise Christensen (2), Sandro D'Odorico (2), Vasily Belokurov (1), N. Wyn Evans (1), Paul C. Hewett (1), Sergey Koposov (3), Elena Mason (2), Joel Vernet (2) ((1) Institute of Astronomy, University of Cambridge, (2) European Southern Observatory, (3) Max Planck Institute for Astronomy)

We have used spectra obtained with X-shooter, the triple arm optical-infrared spectrograph recently commissioned on the Very Large Telescope (VLT) of the European Southern Observatory (ESO), to confirm the gravitational lens nature of the CASSOWARY candidate CSWA 20. This system consists of a luminous red galaxy at redshift z(abs) = 0.741, with a very high velocity dispersion sigma(lens) = 500 km/s, which lenses a blue star-forming galaxy at z(em) = 1.433 into four images with separations of about 3 arcseconds. The source shares many of its properties with those of UV-selected galaxies z = 2-3: it is forming stars at a rate of 25 solar masses per year, has a metallicity of about 1/4 solar, and shows nebular emission from two components separated by 0.4 arcseconds in the image plane, possibly indicating a merger. It appears that foreground interstellar material within the galaxy has been evacuated from the sight-line along which we observe the starburst, giving an unextinguished view of its stars and H II regions. CSWA 20, with its massive lensing galaxy producing a high magnification of an intrinsically luminous background galaxy, is a promising target for future studies at a variety of wavelengths.

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