C. D. Fassnacht

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.

1608+656 - A QUADRUPLE-LENS SYSTEM FOUND IN THE CLASS GRAVITATIONAL LENS SURVEY

The first phase of a large gravitational lens survey using the Very Large Array at a wavelength of 3.6 cm has been completed, yielding images for 3258 radio sources. The Cosmic Lens All-Sky Survey (CLASS) is designed to locate gravitational lens systems consisting of multiply imaged compact components with separations greater than 02. We report here the first discovery of a gravitational lens from the survey: 1608+656, a quadruply imaged object with a maximum separation of 21. Images from the Palomar 5 m and Keck 10 m telescopes show the lensed images and the lensing galaxy. An optical spectrum obtained with the Palomar 5 m telescope indicates a redshift of z=0.6304 for the lensing galaxy. No conclusive redshift for the lensed object has been determined, although a single strong emission line is found at 9240 A in the Keck low-resolution imaging spectrograph spectrum. The two most likely identifications for this line are Hβ (z=0.90) and Mg II (z=2.30). The preliminary lens model derived from the radio image reproduces the observed configuration and relative fluxes of the images, as well as the position, shape, and orientation of the lensing galaxy. Because a simple mass model is able to fit the observations, we argue that this lens system is promising for determining H0.

A determination of H-0 with the class gravitational lens B1608+656. II. Mass models and the Hubble constant from lensing

We present mass models of the four-image gravitational lens system B1608+656, based on information obtained through VLBA imaging, VLA monitoring, and Hubble Space Telescope (HST) WFPC2 and NICMOS imaging. We have determined a mass model for the lens galaxies that reproduces (1) all image positions within the observational errors, (2) two out of three flux-density ratios within about 10% from the observed ratios, and (3) the model time delays within 1% from their observed values, given our best estimate of the Hubble parameter. Using the time delays determined in a companion paper, we also find that the best isothermal mass model gives H0 = 59 km s-1 Mpc-1 for Ωm = 1 and ΩΛ = 0.0, or H0 = (65-63) km s-1 Mpc-1 for Ωm = 0.3 and ΩΛ = 0.0-0.7. The statistical errors indicate the 95.4% (2 σ) confidence interval. A systematic error of ±15 km s-1 Mpc-1 is estimated from a 20% (1 σ) uncertainty in the steepness of radial mass profile. This cosmological determination of H0 agrees well with determinations from three other gravitational lens systems (i.e., B0218+357, Q0957+561, and PKS 1830-211), Type Ia supernovae, the Sunyaev-Zeldovich effect and local determinations. The current agreement on H0—within the 1 σ statistical errors—from four of five gravitational lens systems (1) emphasizes the reliability of its determination from isolated gravitational lens systems and (2) suggests that a close-to-isothermal mass profile can describe disk galaxies (e.g., B0218+357 and possibly PKS 1830-211), ellipticals (e.g., B1608+656), and central cluster ellipticals (e.g., Q0957+561). The average of H0 from B0218+357, Q0957+561, B1608+656, and PKS 1830-211, gives H = 69 ± 7 km s-1 Mpc-1 for a flat universe with Ωm = 1 or H = 74 ± 8 km s-1 Mpc-1 for Ωm = 0.3 and ΩΛ = 0.0-0.7. When including PG 1115+080, these values decrease to 64 ± 11 km s-1 Mpc-1 and 68 ± 13 km s-1 Mpc-1, respectively. The errors are the estimated 2 σ errors on the average. The Hubble parameter from gravitational lenses seems to agree best with local determinations of H0 for a low-density universe, under the assumption that all lenses are nearly isothermal.

B2045+265: A New Four-Image Gravitational Lens from CLASS*

We have discovered a new gravitational lens in the Cosmic Lens All-Sky Survey (CLASS). The lens B2045+265 is a four-image system with a maximum separation of 1.9. A fifth radio component is detected, but its radio spectrum and its positional coincidence with infrared emission from the lensing galaxy strongly suggest that it is the radio core of the lensing galaxy. This implies that the B2045+265 lens system consists of a flat-spectrum radio source that is being lensed by another flat-spectrum radio source. Infrared images taken with the Hubble Space Telescope and the Keck I Telescope detect the lensed images of the background source and the lensing galaxy. The lensed images have relative positions and flux densities that are consistent with those seen at radio wavelengths. The lensing galaxy has magnitudes of J = 19.2, m(F160W) = 18.8, and K = 17.6 mag in a 1.9 diameter aperture, which corresponds to the size of the Einstein ring of the lens. Spectra of the system taken with the Keck I Telescope reveal a lens redshift of z(l) = 0.8673 and a source redshift of z(s) = 1.28. The lens spectrum is typical of an Sa galaxy. The image splitting and system redshifts imply that the projected mass inside the Einstein radius of the lensing galaxy is M(E) = 4.7 x 10(11) h(-1) M(.). An estimate of the light emitted inside the Einstein radius from the K magnitude gives a mass-to-light ratio in the rest-frame B band of (M/L(B))(E) = 20 h (M/L(B))(.). Both the mass and mass-to-light ratio are higher than what is seen in nearby Sa galaxies. In fact, the implied rotation velocity for the lensing galaxy is 2-3 times higher than what is seen in nearby spiral galaxies. The large projected mass inside the Einstein ring radius may be the result of a significant amount of dark matter in the system, perhaps from a compact group of galaxies associated with the primary lensing galaxy; however, it may also arise from a misidentification of the source redshift. A simple model of the gravitational potential of the lens reproduces the image positions well, but further modeling is required to satisfy the constraints from the image flux density ratios. With further observations and modeling, this lens may yield an estimate of H(o).

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.

1608+656: A Gravitationally Lensed PostStarburst Radio Galaxy

The gravitational lens system 1608+656 displays four flat-spectrum, pointlike components that are the images of the unresolved core of a double-lobed radio source. The lensing mass is a galaxy at z = 0.630. New spectra of this system enable us to determine a conclusive redshift of 1.394 for the lensed object. The spectra show prominent high-order Balmer absorption lines and Mg II absorption. These lines, and the absence of [O II] emission, indicate that this is a poststarburst or E + A galaxy. It is unique among lensed objects in not being a quasar and among E + A galaxies in having the highest known redshift. Even allowing for lens magnification, the lensed object is a very luminous galaxy, with an absolute magnitude, M(r) = -22.8 mag. The deconvolved infrared image indicates that the galaxy may be slightly resolved. The radio luminosity density of the lobes is L_(1.4) = 5.78 × 10^(25) W Hz^(-1), which puts the source on the boundary between FR I and FR II radio galaxies. Together with the redshift for the lens and a satisfactory mass model, the determination of the lensed object redshift makes this system an excellent candidate for measuring H_0.

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.

The complex gravitational lens system B1933+503

We report the discovery of the most complex arcsec-scale radio gravitational lens system yet known, B1933+503 was found during the course of the CLASS survey and MERLIN and VLA radio maps reveal up to 10 components. Four of these are compact and have flat spectra: the rest are more extended and have steep spectra, The background lensed object appears to consist of a flat-spectrum core (quadruply imaged) and two compact lobes symmetrically disposed relative to the core, One of the lobes is quadruply imaged while the other is doubly imaged. An HST observation of the system with the WFPC2 shows a galaxy with an axial ratio of 0.5, but none of the images of the background object is detected, A redshift of 0.755 has been measured for the lens galaxy.

A complete infrared Einstein ring in the gravitational lens system B1938+666

We report the discovery, using NICMOS on the Hubble Space Telescope, of an arcsec-diameter Einstein ring in the gravitational lens system B1938 + 666. The lensing galaxy is also detected, and is most likely an early-type galaxy. Modelling of the ring is presented and compared with the radio structure from MERLIN maps. We show that the Einstein ring is consistent with the gravitational lensing of an extended infrared component, centred between the two radio components.