CASSOWARY 20: a wide separation Einstein Cross identified with the X-shooter spectrograph

We have used spectra obtained with X-shooter, the triple arm optical-infrared spectrograph recently commissioned on the Very Large Telescope of the European Southern Observatory, to confirm the gravitational lens nature of the CAmbridge Sloan Survey Of Wide ARcs in the skY (CASSOWARY) candidate CSWA 20. This system consists of a luminous red galaxy at redshift z abs = 0.741, with a very high velocity dispersion, σ lens ≃ 500 km s -1 , which lenses a blue star-forming galaxy at z em = 1.433 into four images with a mean separation of ~6 arcsec. The source shares many of its properties with those of UV-selected galaxies at z = 2-3: it is forming stars at a rate SFR ≃ 25 M ⊙ yr -1 , has a metallicity of ~1/4 solar and shows nebular emission from two components separated by 0.4 arcsec (in the image plane), possibly indicating a merger. It appears that foreground interstellar material within the galaxy has been evacuated from the sightline 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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