Keck spectroscopy of Cosmic Lens All-Sky Survey gravitational lenses

Abstract

We present the optical spectra of four newly discovered gravitational lenses from the Cosmic Lens All-Sky Survey (CLASS). These observations were carried out using the Low-Resolution Imaging Spectrograph on the W. M. Keck-I Telescope as part of a programme to study galaxy-scale gravitational lenses. From our spectra, we found the redshift of the background source in CLASS B0128+437 (z s = 3.1240 ± 0.0042) and the lensing galaxy redshifts in CLASS B0445+123 (z 1 = 0.5583 ± 0.0003) and CLASS B0850+054 (z 1 = 0.5883 ± 0.0006). Intriguingly, we also discovered that CLASS B0631+519 may have two lensing galaxies (z 1,1 = 0.0896 ± 0.0001, z 1,2 = 0.6196 ± 0.0004). We also found a single unidentified emission line from the lensing galaxy in CLASS B0128+437 and the lensed source in CLASS B0850+054. We find the lensing galaxies in CLASS B0445+123 and CLASS B0631+519 (1, 2) to be early-type galaxies with Einstein radii of 2.8-3.0 h -1 kpc. The deflector in CLASS B0850+054 is a late-type galaxy with an Einstein radius of 1.6 h -1 kpc.