X-shooter Observations of the Gravitational Lens System CASSOWARY 5

TL;DR

This paper reports the confirmation and initial analysis of a new gravitational lens system, CSWA5, revealing a young, star-forming galaxy at z=1.0686 lensed by a foreground galaxy group, with detailed spectral properties.

Contribution

First spectroscopic confirmation of the CSWA5 gravitational lens system and initial characterization of the lensed galaxy’s physical properties.

Findings

Galaxy is young, 25-50 Myr old

Star-formation rate ~20 solar masses/yr

Stellar mass ~3 billion solar masses

Abstract

We confirm an eighth gravitational lens system in the CASSOWARY catalogue. Exploratory observations with the X-shooter spectrograph on the VLT show the system CSWA5 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 approximately 20 solar masses/yr, the galaxy has already assembled a stellar mass of 3 x 10^9 solar masses 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 CSWA5 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.