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

Abstract

We confirm an eighth gravitational lens system in the CASSOWARY catalogue.\nExploratory observations with the X-shooter spectrograph on the VLT show the\nsystem CSWA5 to consist of at least three images of a blue star-forming galaxy\nat z = 1.0686, lensed by an apparent foreground group of red galaxies one of\nwhich is at z = 0.3877. The lensed galaxy exhibits a rich spectrum with broad\ninterstellar absorption lines and a wealth of nebular emission lines.\nPreliminary analysis of these features shows the galaxy to be young, with an\nage of 25-50 Myr. With a star-formation rate of approximately 20 solar\nmasses/yr, the galaxy has already assembled a stellar mass of 3 x 10^9 solar\nmasses and reached half-solar metallicity. Its blue spectral energy\ndistribution and Balmer line ratios suggest negligible internal dust\nextinction. A more in-depth analysis of the properties of this system is\ncurrently hampered by the lack of a viable lensing model. However, it is\nalready clear that CSWA5 shares many of its physical characteristics with the\ngeneral population of UV-selected galaxies at redshifts z = 1-3, motivating\nfurther study of both the source and the foreground mass concentration\nresponsible for the gravitational lensing.\n