P. M. Phillips

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

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/ .

The Cosmic Lens All-Sky Survey - II. Gravitational lens candidate selection and follow-up

We report the final results of the search for gravitationally lensed flat-spectrum radio sources found in the combination of CLASS (Cosmic Lens All-Sky Survey) and JVAS (Jodrell Bank VLA Astrometric Survey). VLA (Very Large Array) observations of 16 503 sources have been made, resulting in the largest sample of arcsec-scale lens systems available. Contained within the 16 503 sources is a complete sample of 11 685 sources which have two-point spectral indices between 1.4 and 5 GHz flatter than −0.5, and 5-GHz flux densities 30 mJy. A subset of 8958 sources form a well-defined statistical sample suitable for analysis of the lens statistics. We describe the systematic process by which 149 candidate lensed sources were picked from the statistical sample on the basis of possessing multiple compact components in the 0.2-arcsec resolution VLA maps. Candidates were followed up with 0.05-arcsec resolution MERLIN and 0.003-arcsec VLBA observations at 5 GHz and rejected as lens systems if they failed well-defined surface brightness and/or morphological tests. To illustrate the candidate elimination process, we show examples of sources representative of particular morphologies that have been ruled out by the follow-up observations. 194 additional candidates, not in the well-defined sample, were also followed up. Maps for all the candidates can be found on the World Wide Web at http://www.jb.man.ac.uk/research/gravlens/index.html. We summarize the properties of each of the 22 gravitational lens systems in JVAS/CLASS. 12 are double-image systems, nine are four-image systems and one is a six-image system. 13 constitute a statistically well-defined sample giving a point-source lensing rate of 1:690 ± 190. The interpretation of the results in terms of the properties of the lensing galaxy population and cosmological parameters will be published elsewhere.

Constraints on cosmological parameters from the analysis of the Cosmic Lens All Sky Survey radio-selected gravitational lens statistics

We derive constraints on cosmological parameters and the properties of the lensing galaxies from gravitational lens statistics based on the final Cosmic Lens All Sky Survey data. For a flat universe with a classical cosmological constant, we find that the present matter fraction of the critical density is Omega(m)=0.31(+0.27)(-0.14) (68%)+0.12-0.10 (syst). For a flat universe with a constant equation of state for dark energy w=p(x)(pressure)/rho(x)(energy density), we find w<-0.55(+0.18)(-0.11) (68%).

Radio, optical and infrared observations of CLASS B0128+437

We present new observations of the gravitational lens system CLASS B0128+437 made in the optical, infrared and radio regimes. Hubble Space Telescope observations detect only a very faint, extended object in the I band with no obvious emission from the lensed images visible; no detection at all is made in the V band. The lens system is detected with a much higher signal-to-noise ratio with the United Kingdom Infrared Telescope in the K band and, although resolved, the resolution is not sufficient to allow the lensed images and the lens galaxy to be separated. A careful astrometric calibration, however, suggests that the peak of the infrared emission corresponds to the two merging images A and B and therefore that the lensed images dominate at infrared wavelengths. The new radio data consist of high-resolution very long baseline interferometry radio images at three frequencies, 2.3, 5 and 8.4 GHz, made with the Very Long Baseline Array (VLBA) and the 100-m Effelsberg telescope. These reveal that the lensed source consists of three well-defined subcomponents that are embedded in a more extended jet. Due to the fact that the subcomponents have different spectral indices, it is possible to determine, unambiguously, which part of each image corresponds to the same source subcomponent. Our main finding is that one of the images, B, looks very different to the others, there being no obvious division into separate subcomponents and the image being apparently both broader and smoother. This is a consequence, we believe, of scatter-broadening in the interstellar medium of the lensing galaxy. The large number of multiply imaged source subcomponents also provides an abundance of modelling constraints and we have attempted to fit a singular isothermal ellipsoid + external shear model to the data, as well as utilizing the novel method of Evans and Witt. It proves difficult in both cases, however, to obtain a satisfactory fit, which strongly suggests the presence of substructure in the mass distribution of the lensing galaxy, perhaps of the kind that is predicted by cold dark matter theories of structure formation.

CLASS B2108+213: a new wide-separation gravitational lens system

We present observations of CLASS B2108+213, the widest separation gravitational lens system discovered by the Cosmic Lens All-Sky Survey. Radio imaging using the VLA at 8.46 GHz and MERLIN at 5 GHz shows two compact components separated by 4.56 arcsec with a faint third component in between which we believe is emission from a lensing galaxy. 5-GHz VLBA observations reveal milliarcsecond-scale structure in the two lensed images that is consistent with gravitational lensing. Optical emission from the two lensed images and two lensing galaxies within the Einstein radius is detected in Hubble Space Telescope imaging. Furthermore, an optical gravitational arc, associated with the strongest lensed component, has been detected. Surrounding the system are a number of faint galaxies which may help explain the wide image separation. A plausible mass distribution model for CLASS B2108+213 is also presented.

B0850+054 : a new gravitational lens system from CLASS

We report the discovery of a new gravitational lens system from the CLASS survey. Radio observations with the VLA, WSRT and MERLIN show that the radio source B0850+054 is composed of two compact components with identical spectra, a separation of 0.7 arcsec and a flux density ratio of 6 : 1. VLBA observations at 5 GHz reveal structures that are consistent with the gravitational lens hypothesis. The brighter of the two images is resolved into a linear string of at least six subcomponents, whilst the weaker image is radially stretched towards the lens galaxy. UKIRT K -band imaging detects an 18.7-mag extended object, but the resolution of the observations is not sufficient to resolve the lensed images and the lens galaxy. Mass modelling has not been possible with the present data and the acquisition of high-resolution optical data is a priority for this system.

CLASS B0631+519: Last of the cosmic lens all-sky survey lenses

We report the discovery of a new gravitational lens system from the CLASS survey, CLASS B0631+519. VLA, MERLIN and VLBA observations show a doubly-imaged radio core, a doubly-imaged lobe and a second lobe that is probably quadruply-imaged. The maximum image separation is 1.16 arcseconds. The VLBA resolves the most magnified image of the flat-spectrum radio core into a number of sub-components spread across approximately 20 milli-arcseconds. Optical and near-infrared imaging with the ACS and NICMOS cameras on the HST show that there are two galaxies along the line of sight to the lensed source, as was previously discovered by optical spectroscopy. The nearer galaxy at z=0.0896 is a small blue irregular, while the more distant galaxy at z=0.6196 is an elliptical type and appears to contribute most of the lensing effect. The host galaxy of the lensed source is visible in NICMOS imaging as a set of arcs that form an almost complete Einstein ring. Mass modelling using non-parametric techniques can reproduce the ring and indicates that the irregular galaxy has a (localised) effect on the flux density distribution in the Einstein ring at the 5-10% level.

CLASS B0445+123: A new two-image gravitational lens system

A new two-image gravitational lens system has been discovered as a result of the Cosmic Lens All-Sky Survey. Radio observations with the Very Large Array (VLA), the MultiElement Radio Linked Interferometer Network and the Very Long Baseline Array at increasingly higher resolutions all show two components with a flux density ratio of similar to7 : 1 and a separation of 1.34 arcsec. Both components are compact and have the same spectral index. Follow-up observations made with the VLA at 8.4 GHz show evidence of a feature to the south-east of the brighter component and a corresponding extension of the weaker component to the north-west. Optical observations with the William Herschel Telescope show similar to1.7-arcsec extended emission aligned in approximately the same direction as the separation between the radio components with an R -band magnitude of 21.8 +/- 0.4.

Class B0631+519: Last of the Class Lenses

We report the discovery of a new gravitational lens system from the CLASS survey, CLASS B0631+519. VLA, MERLIN and VLBA observations show a doubly-imaged radio core, a doubly-imaged lobe and a second lobe that is probably quadruply-imaged. The maximum image separation is 1.16 arcseconds. The VLBA resolves the most magnified image of the flat-spectrum radio core into a number of sub-components spread across approximately 20 milli-arcseconds. Optical and near-infrared imaging with the ACS and NICMOS cameras on the HST show that there are two galaxies along the line of sight to the lensed source, as was previously discovered by optical spectroscopy. The nearer galaxy at z=0.0896 is a small blue irregular, while the more distant galaxy at z=0.6196 is an elliptical type and appears to contribute most of the lensing effect. The host galaxy of the lensed source is visible in NICMOS imaging as a set of arcs that form an almost complete Einstein ring. Mass modelling using non-parametric techniques can reproduce the ring and indicates that the irregular galaxy has a (localised) effect on the flux density distribution in the Einstein ring at the 5-10% level.

CLASS B0445+123 : a new two-image gravitational lens system

We derive constraints on cosmological parameters and the properties of the lensing galaxies from gravitational lens statistics based on the final Cosmic Lens All Sky Survey data. For a flat universe with a classical cosmological constant, we find that the present matter fraction of the critical density is Omega(m)=0.31(+0.27)(-0.14) (68%)+0.12-0.10 (syst). For a flat universe with a constant equation of state for dark energy w=p(x)(pressure)/rho(x)(energy density), we find w<-0.55(+0.18)(-0.11) (68%).