The Cosmic Lens All-Sky Survey - I. Source selection and observations

Abstract

The Cosmic Lens All-Sky Survey (CLASS) is an international collaborative program which has obtained high-resolution radio images of over 10000 flat-spectrum radio sources in order to create the largest and best studied statistical sample of radioloud gravitationally lensed systems. With this survey, combined with detailed studies of the lenses found therein, constraints can be placed on the expansion rate, matter density, and dark energy (e.g. cosmological constant, quintessence) content of the Universe that are complementary to and independent of those obtained through other methods. CLASS is aimed at identifying lenses where multiple images are formed from compact flat-spectrum radio sources, which should be easily identifiable in the radio maps. Because CLASS is radio-based, dust obscuration in lensing galaxies is not a factor, and the relative insensitivity of the instrument to environmental conditions (e.g. weather, “seeing”) leads to nearly uniform sensitivity and resolution over the entire survey. In four observing “seasons” from 1994–1999, CLASS has observed 13783 radio sources with the VLA at 8.4 GHz in its largest “A” configuration (0. ′′ 2 resolution). When combined with the JVAS survey, the CLASS sample contains over 16,000 images. A complete sample of 11685 sources was observed, selected to have a flux density of at least 30 mJy in the GB6 catalogue at 4.85 GHz (spanning the declination range 0 ◦ 6 � 6 75 ◦ and |b| > 10 ◦ , excluding the galactic plane) and a spectral index � > 0.5 between the NVSS at 1.4 GHz and the GB6. A typical 30second CLASS snapshot reached an rms noise level of 0.4 mJy. So far, CLASS has found 16 new gravitational lens systems, and the JVAS/CLASS survey contains a total of 22 lenses. The follow-up of a small number of candidates using the VLA, MERLIN, the VLBA, and optical telescopes is still underway. In this paper, we present a summary of the CLASS observations, the JVAS/CLASS sample, and statistics on sub-samples of the survey. A companion paper presents the lens candidate selection and in a third paper the implications for cosmology are discussed. The source catalogues from the JVAS/CLASS project described in this paper are available from http://www.jb.man.ac.uk/research/gravlens/ .