P. N. Wilkinson

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 First Caltech-Jodrell Bank VLBI Survey. III. VLBI and MERLIN Observations at 5 GHz and VLA Observations at 1.4 GHz

We present the 5 GHz results from the first Caltech-Jodrell Bank (CJ1) VLBI survey. The 1.6 GHz maps were presented in two separate papers (Polatidis et al. 1995; Thakkar et al. 1995). These three papers complete the first stage of this program to map at both 1.6 and 5 GHz all objects accessible to Mark II VLBI in the complete sample of 135 objects with 1.3 > S_(5 GHz) ≥ 0.7 Jy, δ(1950) ≥ 35°, and |b| > 10°. The combination of the CJ1 sample with the Pearson-Readhead (PR) sample provides a complete, flux density-limited sample of 200 objects with S_(5 GHz) ≥ 0.7 Jy, δ(1950) ≥35°, and |b| > 10° for which all of the objects accessible to Mark II VLBI have been mapped at both 5 GHz(129 objects) and 1.6 GHz(132 objects). In addition to the 5 GHz VLBI maps, we present in this paper 5 GHz MERLIN observations of 20 objects and 1.4 GHz VLA observations of 92 objects in the combined CJ1 + PR sample. The VLA maps, together with L- band (1.3-1.7 GHz) maps available in the literature, provide a complete set of VLA maps for the combined CJ1 + PR sample. Finally, we present the radio spectra of the objects in the CJ1 sample. The combined CJ1 + PR VLBI surveys provide a sample which is large enough for a number of important astrophysical and cosmological studies. These will be presented in further papers in this series.

High-resolution studies of radio sources in the Hubble Deep and Flanking Fields

18 days of MERLIN data and 42 h of A-array VLA data at 1.4 GHz have been combined to image a 10-arcmin field centred on the Hubble Deep Field (HDF). This area also includes the Hubble Flanking Fields (HFF). A complete sample of 92 radio sources with S 1.4 > 40 μJy was detected using the VLA data alone and then imaged with the MERLIN+VLA combination. The combined images offer (i) higher angular resolution (synthesized beams of diameter 0.2-0.5 arcsec), (ii) improved astrometric accuracy, and (iii) improved sensitivity compared with VLA-only data. The images are amongst the most sensitive yet made at 1.4GHz, with rms noise levels of 3.3 μJy beam -1 in the 0.2-arcsec images. Virtually all the sources are resolved, with angular sizes in the range 0.2 to 3 arcsec. The central 3-arcmin square was imaged separately to search for sources down to 27 μJy. No additional sources were detected, indicating that sources fainter than 40 μJy are heavily resolved with MERLIN and must have typical angular sizes > 0.5 arcsec. Radio sources associated with compact galaxies have been used to align the HDF, the HFF and a larger CFHT optical field to the radio-based International Celestial Reference Frame. The HST optical fields have been registered to 70 per cent of the sources are starburst-type systems associated with major disc galaxies in the redshift range 0.3-1.3. Chandra detections are associated with 55 of the 92 radio sources, but their X-ray flux densities do not appear to be correlated with the radio flux densities or morphologies. The most recent submillimetre results on the HDF and HFF do not provide any unambiguous identifications with these latest radio data, except for HDF8550.1, but suggest at least three strong candidates.

A determination of H-0 with the class gravitational lens B1608+656. I. Time delay measurements with the VLA

We present the results of a program to monitor the four-image gravitational lens B1608+656 with the VLA. The system was observed over a 7 month period from 1996 October to 1997 May. The 64 epochs of observation have an average spacing of 3.6 days. The light curves of the four images of the background source show that the flux density of the background source has varied at the ~5% level. We measure time delays in the system based on common features that are seen in all four light curves. The three independent time delays in the system are found to be ΔtBA = 31 ± 7 days, ΔtBC = 36 ± 7 days, and ΔtBD = 76 days at 95% confidence. The uncertainties on the time delays are determined by Monte Carlo simulations that use fake light curves that have the characteristics of the observed light curves. This is the first gravitational lens system for which three independent time delays have been measured. A companion paper presents a mass model for the lensing galaxy that correctly reproduces the observed image positions, flux density ratios, and time delay ratios. The last condition is crucial for determining H0 with a four-image lens. We combine the time delays with the model to obtain a value for the Hubble constant of H0 = 59 km s-1 Mpc-1 at 95% confidence (statistical) for (ΩM,ΩΛ) = (1,0). In addition, there is an estimated systematic uncertainty of ±15 km s-1 Mpc-1 from uncertainties in modeling the radial mass profiles of the lensing galaxies. The value of H0 presented in this paper is comparable to recent measurements of H0 from the gravitational lenses 0957+561, PG 1115+080, B0218+357, and PKS 1830-211.

The second Caltech-Jodrell Bank VLBI survey. 1: Observations of 91 of 193 sources

We define the sample for the second Caltech-Jodrell Bank very long base interferometry (VLBI) survey. This is a sample of 193 flat- or gigahertz-peaked-spectrum sources selected at 4850 MHz. This paper presents images of 91 sources with a resolution of approximately 1 mas, obtained using VLBI observations at 4992 MHz with a global array. The remaining images and the integrated radio spectra will be presented in a forthcoming paper by Henstock et al.

The Second Caltech--Jodrell Bank VLBI Survey. II. Observations of 102 of 193 Sources

This is the second of two papers presenting the Second Caltech-Jodrell Bank VLBI survey (CJ2). The CJ2 sample consists of 193 flat- and gigahertz-peaked-spectrum sources selected at 4850 MHz. In this paper we present images of the remaining 102 sources with a resolution of ~1 mas, obtained using VLBi snapshot observations at 4992 MHz with a global array. We also present integrated radio spectra for the entire CJ2 sample.

Bent beams and the overall size of extragalactic radio sources

In a small sample of extragalactic radio sources, curvature is found to be a common feature in bright radio components coincident with quasi-stellar objects. In 3C273 and 3C345 the curvature is smooth between the nuclear (<0.003″) and the outer (>1″) regions. This contrasts sharply with objects having comparatively faint central radio components, which are often well aligned with the extended features.

EXTRINSIC RADIO VARIABILITY OF JVAS/CLASS GRAVITATIONAL LENSES

We present flux ratio curves of the fold and cusp (i.e., close multiple) images of six Jodrell Bank VLA Astrometric Survey and Cosmic Lens All-Sky Survey (JVAS/CLASS) gravitational lens systems. The data were obtained over a period of 8.5 months in 2001 with the Multi Element Radio- Linked Interferometer Network ( MERLIN) at 5 GHz with 50 mas resolution, as part of a MERLIN Key Project. Even though the time delays between the fold and cusp images are small (less than or similar to1 day) compared to the timescale of intrinsic source variability, all six lens systems show evidence that suggests the presence of extrinsic variability. In particular, the cusp images of B2045+ 265 - regarded as the strongest case of the violation of the cusp relation (i.e., the sum of the magnifications of the three cusp images add to zero) - show extrinsic variations in their flux ratios up to similar to40% peak to peak on timescales of several months. Its low Galactic latitude of b approximate to - 10degrees and a line of sight toward the Cygnus superbubble region suggest that Galactic scintillation is the most likely cause. The cusp images of B1422+ 231 at b approximate to +69degrees do not show strong extrinsic variability. Galactic scintillation can therefore cause significant scatter in the cusp and fold relations of some radio lens systems ( up to 10% rms), even though these relations remain violated when averaged over a less than or similar to 1 yr time baseline.

CLASS B1152+199 and B1359+154: Two New Gravitational Lens Systems Discovered in the Cosmic Lens All-Sky Survey

The third phase of the Cosmic Lens All-Sky Survey (CLASS) has recently been completed, bringing the total number of sources imaged to over 15,000 in the CLASS and Jodrell-VLA Astrometric Survey combined survey. In the VLA observations carried out in 1998 March and April, two new candidate lensed systems were discovered: CLASS B1152+199 and B1359+154. B1152+199 is a 16 double, with a background quasar at z = 1.019 lensed by a foreground galaxy at z = 0.439. The relatively flat radio spectra of the lensed images (α = -0.32), combined with a previous ROSAT detection of the source, make B1152+199 a strong candidate for time delay studies at both radio and X-ray wavelengths. B1359+154 is a quadruply lensed quasar at z = 3.235, with a maximum image separation of 17. As yet, the redshift of the lensing object in this system is undetermined. The steep spectral index of the source (α = -0.9) suggests that B1359+154 will not exhibit strong variability and is therefore unlikely to be useful for determining H0 from measured time delays.

A multi-epoch VLBI survey of the kinematics of CFJ sources. II. Analysis of the kinematics

Context. This is the second of a series of papers presenting VLBI observations of the 293 Caltech-Jodrell Bank Flat-Spectrum (hereafter CJF) sources and their analysis. Aims. To obtain a consistent motion dataset large enough to allow the systematic properties of the population to be studied. Methods. We present the detailed kinematic analysis of the complete fl ux-density limited CJF survey. We computed 2-D kinematic models based on the optimal model-fitting parameters of mult i-epoch VLBA observations. This allows us to calculate not only radial, but also orthogonal motions, and thus to study curvature and acceleration. Statistical tests of the motions measured an d their reliability have been performed. A correlation analysis between the derived apparent motions, luminosities, spectral indices, an d core dominance and the resulting consequences is described. Results. With at least one velocity in each of 237 sources, this sample is much larger than any available before and allows a meaningful statistical investigation of apparent motions and any poss ible correlations with other parameters in AGN jets. The main results to emerge are as follows: - In general motions are not consistent with a single uniform velocity applicable to all components along a jet. - We find a slight trend towards a positive outward accele ration and also adduce some evidence for greater acceleration in the inner-most regions. - We find a lack of fast components at phys ical distances less than a few pc from the reference feature. - Only ∼4% of the components from galaxies and<2% of those from quasars undergo large bends i.e. within 15 ◦ of±90 ◦ . - The distribution of radial velocities shows a broad distribution of velociti es (apparent velocities up to 30 c). 15% of the best sampled jet components exhibit ”low” velocities which may need to be explained in a different manner to the ”fast” motions. - Some negative or ”backwards” superluminal motions are seen and in 15 cases (6%) these are definitely significant. - We find a strong correlation between t he 5 GHz luminosity and the apparent velocity. - The CJF galaxies, on average, show slower apparent jet component velocities than the quasars. - The mean velocity in the VLBA 2cm survey (Kellermann et al. 2004) is substantially higher than in the CJF survey ‐ the rati o could be roughly a factor of 1.5‐2. This supports the observed trend of increasing apparent velocity with increasing observing frequency. Conclusions. This AGN survey provides the basis for any statistical analysis of jet and jet-component properties.