X-shooter observations of the gravitational lens system CASSOWARY 5★

We confirm an eighth gravitational lens system in the CAmbridge Sloan Survey Of Wide ARcs in the skY (CASSOWARY) catalogue. Exploratory observations with the X-shooter spectrograph on the Very Large Telescope of the European Southern Observatory show the system CASSOWARY 5 (CSWA 5) to consist of at least three images of a blue star-forming galaxy at z = 1.0686, lensed by an apparent foreground group of red galaxies, one of which is at z = 0.3877. The lensed galaxy exhibits a rich spectrum with broad interstellar absorption lines and a wealth of nebular emission lines. Preliminary analysis of these features shows the galaxy to be young, with an age of ~25-50 Myr. With a star formation rate of ~20 M ⊙ yr -1 , the galaxy has already assembled a stellar mass M * ~ 3 × 10 9 M ⊙ and reached half-solar metallicity. Its blue spectral energy distribution and Balmer line ratios suggest negligible internal dust extinction. A more in-depth analysis of the properties of this system is currently hampered by the lack of a viable lensing model. However, it is already clear that CSWA 5 shares many of its physical characteristics with the general population of UV-selected galaxies at redshifts z = 1-3, motivating further study of both the source and the foreground mass concentration responsible for the gravitational lensing.

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