A. D. Biggs

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.

Time delay for the gravitational lens system B0218+357

Measurement of the time delay between multiple images of a gravitational lens system is potentially an accurate method of determining the Hubble constant over cosmological distances. One of the most promising candidates for an application of this technique is the system B0218+357, which was found in the Jodrell Bank/VLA Astrometric Survey (JVAS). This system consists of two images of a compact radio source, separated by 335 milliarcsec, and an Einstein ring which can provide a strong constraint on the mass distribution in the lens. We present here the results of a three-month VLA monitoring campaign at two frequencies. The data are of high quality, and both images show clear variations in total flux density, percentage polarization and polarization position angle at both frequencies. The time delay between the variations in the two images has been calculated using a chi-squared minimization to be 10.5 +/- 0.4 d at 95 per cent confidence, with the error being derived from Monte Carlo simulations of the light curves. Although mass modelling of the system is at a preliminary stage, taking the lensing galaxy to be a singular isothermal ellipsoid and using the new value for the time delay, we obtain a value for the Hubble constant of 69(-19)(+13) km s(-1) Mpc(-1), again at 95 per cent confidence.

Models for the lens and source of B0218+357: a LensClean approach to determine H0

B0218+357 is one of the most promising systems to determine the Hubble constant from time-delays in gravitational lenses. Consisting of two bright images, which are well resolved in very long baseline interferometry (VLBI) observations, plus one of the most richly structured Einstein rings, it potentially provides better constraints for the mass model than most other systems. The main problem left until now was the very poorly determined position of the lensing galaxy. After presenting detailed results from classical lens modelling, we apply our improved version of the LensClean algorithm which for the first time utilizes the beautiful Einstein ring for lens modelling purposes. The primary result using isothermal lens models is a now very well defined lens position of (255 ± 6, 119 ± 4) mas relative to the A image, which allows the first reliable measurement of the Hubble constant from the time-delay of this system. The result of H0= (78 ± 6) km s−1 Mpc−1 (2σ) is very high compared with other lenses. It is, however, compatible with local estimates from the Hubble Space Telescope (HST) key project and with WMAP results, but less prone to systematic errors. We furthermore discuss possible changes of these results for different radial mass profiles and find that the final values cannot be very different from the isothermal expectations. The power-law exponent of the potential is constrained by VLBI data of the compact images and the inner jet to be β= 1.04 ± 0.02, which confirms that the mass distribution is approximately isothermal (corresponding to β= 1), but slightly shallower. The effect on H0 is reduced from the expected 4 per cent decrease by an estimated shift of the best galaxy position of circa 4 mas to at most 2 per cent. Maps of the unlensed source plane produced from the best LensClean brightness model show a typical jet structure and allow us to identify the parts which are distorted by the lens to produce the radio ring. We also present a composite map which for the first time shows the rich structure of B0218+357 on scales ranging from mas to arcsec, both in the image plane and in the reconstructed source plane. Finally, we use a comparison of observations at different frequencies to investigate the question of possible weakening of one of the images by propagation effects and/or source shifts with frequency. The data clearly favour the model of significant ‘extinction’ without noticeable source position shifts. The technical details of our variant of the LensClean method are presented in the accompanying Paper I.

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%).

High-resolution observations and mass modelling of the CLASS gravitational lens B1152+199

We present a series of high-resolution radio and optical observations of the CLASS gravitational lens system B1152+199 obtained with the Multi-Element Radio-Linked Interferometer Network, Very Long Baseline Array and Hubble Space Telescope. Based on the milliarcsecond-scale substructure of the lensed radio components and precise optical astrometry for the lensing galaxy, we construct models for the system and place constraints on the galaxy mass profile. For a single galaxy model with surface mass density Σ(r)∝r−β, we find that 0.95β1.21 at 2σ confidence. Including a second deflector to represent a possible satellite galaxy of the primary lens leads to slightly steeper mass profiles.

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.

Global 8.4-GHz VLBI observations of JVAS B0218+357

In this paper we present new observations of the gravitational lens system JVAS B0218+357 made with a global very long baseline interferometry (VLBI) network at a frequency of 8.4 GHz. Our maps have an rms noise of 30 μJy beam - 1 and with these we have been able to image much of the extended structure of the radio jet in both the A and B images at high resolution (∼1 mas). The main use of these maps will be to enable us to further constrain the lens model for the purposes of H 0 determination. We are able to identify several subcomponents common to both images with the expected parity reversal, including one which we identify as a counter-jet. We have not been successful in detecting either the core of the lensing galaxy or a third image. Using a model of the lensing galaxy we have back-projected both of the images to the source plane and find that they agree well. However, there are small, but significant, differences which we suggest may arise from multi-path scattering in the interstellar medium (ISM) of the lensing galaxy. We also find an exponent of the radial mass distribution of β 1.04, in agreement with lens modelling of published 15-GHz VLBI data. Polarization maps of each image are presented which show that the distributions of polarization across images A and B are different. We suggest that this results from Faraday rotation and associated depolarization in the lensing galaxy.

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.

MERLIN/VLA imaging of the gravitational lens system B0218+357

Gravitational lenses offer the possibility of accurately determining the Hubble parameter (H_0) over cosmological distances, and B0218+357 is one of the most promising systems for an application of this technique. In particular this system has an accurately measured time delay (10.5+/-0.4 d; Biggs et al. 1999) and preliminary mass modelling has given a value for H_0 of 69 +13/-19 km/s/Mpc. The error on this estimate is now dominated by the uncertainty in the mass modelling. As this system contains an Einstein ring it should be possible to constrain the model better by imaging the ring at high resolution. To achieve this we have combined data from MERLIN and the VLA at a frequency of 5 GHz. In particular MERLIN has been used in multi-frequency mode in order to improve substantially the aperture coverage of the combined data set. The resulting map is the best that has been made of the ring and contains many new and interesting features. Efforts are currently underway to exploit the new data for lensing constraints using the LensClean algorithm (Kochanek & Narayan 1992).