CASSOWARY 20: a Wide Separation Einstein Cross Identified with the X-shooter Spectrograph

TL;DR

This paper confirms the gravitational lensing nature of CASSOWARY 20 using X-shooter spectra, revealing a high-redshift star-forming galaxy magnified by a massive foreground galaxy, enabling detailed study of its properties.

Contribution

First spectroscopic confirmation of CASSOWARY 20 as a gravitational lens system with detailed analysis of the background galaxy and lensing galaxy properties.

Findings

Lensing galaxy at z=0.741 with sigma=500 km/s

Background galaxy at z=1.433 forming stars at 25 solar masses/year

Evidence of a possible merger in the background galaxy

Abstract

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 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 mean separation of about 6 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.