Last update: 27 Oct 2005

The CFHT AO image of CLASS B1359+154 showing the lensed images as well as 4 of the at least 6 possible lensing objects! (D.Rusin et al. 1999)

Quick CLASS links:

What's New:

My CLASS and lensing related publications:

York, T. et al. (2005) CLASS B0631+519: last of the Cosmic Lens All-Sky Survey lenses MNRAS, 361, 259-271 (2005); astro-ph/0505093
G.B. Taylor, et al. (2005) VLBA Imaging Polarimetry of Active Galactic Nuclei: An Automated Approach ApJS, 159, 27-40 (2005); astro-ph/0503234
McKean, J.P., et al. (2005) CLASS B2108+213: a new wide-separation gravitational lens system MNRAS, 356, 1009-1016 (2005); astro-ph/0410554
Myers, S.T., et al. (2003) The Cosmic Lens All-Sky Survey - I. Source selection and observations MNRAS, 341, 1-12 (2003); astro-ph/0211073
Browne, I.W.A, et al. (2003) The Cosmic Lens All-Sky Survey - II. Gravitational lens candidate selection and follow-up MNRAS, 341, 13-32 (2003); astro-ph/0211069
Biggs, A.D., et al. (2003) B0850+054: a new gravitational lens system from CLASS MNRAS, 338, 1084-1088 (2003); astro-ph/0210504
Argo, M.K., et al. (2003) CLASS B0445+123: a new two-image gravitational lens system MNRAS, 338, 957-961 (2003); astro-ph/0210234
Chae, K.-H., et al. (2002) Constraints on Cosmological Parameters from the Analysis of the Cosmic Lens All Sky Survey Radio-Selected Gravitational Lens Statistics PRL, 89, 151301 (2002); astro-ph/0209602
Rusin, D., et al. (2002) High-resolution observations and mass modelling of the CLASS gravitational lens B1152+199 MNRAS, 330, 205-211 (2002); astro-ph/0110099
Rusin, D., et al. (2001) The New Two-Image Gravitational Lens System CLASS B2319+051 AJ, 122, 591-597 (2001); astro-ph/0104399
Marlow, D.R., et al. (2001) CLASS B0739+366: A New Two-Image Gravitational Lens System AJ, 121, 619-624 (2001); astro-ph/0008037
Phillips, P.M., et al. (2000) A new quadruple gravitational lens system: CLASS B0128+437 MNRAS, 319, L7-L11 (2000); astro-ph/0009334
Koopmans, L.V.E., et al. (2000) CLASS B0827+525: `Dark lens' or binary radio-loud quasar? A&A, 361, 815-822 (2000); astro-ph/0007286
Rusin, D., et al. (2000) Adaptive Optics Imaging of the CLASS Gravitational Lens System B1359+154 with the Canada-France-Hawaii Telescope ApJL, 533, L89-L92 (2000); astro-ph/99911420
Fassnacht, C.D., et al. (1999) A Determination of H0 with the CLASS Gravitational Lens B1608+656. I. Time Delay Measurements with the VLA ApJ, 527, 498-512 (1999); astro-ph/99907257
Marlow, D.R., et al. (1999) CLASS B1555+375: A New Four-Image Gravitational Lens System AJ, 118, 654-658 (1999)
Myers, S.T., et al. (1999) CLASS B1152+199 and B1359+154: Two New Gravitational Lens Systems Discovered in the Cosmic Lens All-Sky Survey AJ, 117, 2565-2572 (1999); astro-ph/99905043
Myers, S.T. (1999) Scaling the universe: Gravitational lenses and the Hubble constant Proc. Natl. Acad. Sci. 96, 4236-4239 (1999)
Koopmans, L.V.E., et al. (1999) A new radio double lens from CLASS: B1127+385 MNRAS, 303, 727-735 (1999); astro-ph/99811425
Fassnacht, C.D., et al. (1999) B2045+265: A New Four-Image Gravitational Lens from CLASS AJ, 117, 658-670 (1999); astro-ph/99811167
Sykes, C.M., et al. (1998) The complex gravitational lens system B1933+503 MNRAS, 301, 310-314 (1998) astro-ph/9710358
Jackson, N., et al. (1998) B0712+472: a new radio four-image gravitational lens MNRAS, 296, 483-490 (1998)
Myers, S.T., et al. (1995) 1608+656: A Quadruple-Lens System Found in the CLASS Gravitational Lens Survey ApJL, 447, L5-L8 (1995)
Snellen, I.A.G., et al. (1995) Radio Observations of the Quadruple Lens 1608+656 ApJL, 447, L9-L12 (1995)
Jackson, N., et al. (1995) 1600+434: a new gravitational lens system MNRAS, 274, L25-L29 (1995)


The Cosmic Lens All-Sky Survey (CLASS), is an international (USA, UK and Netherlands) collaborative project to map more than 10,000 radio sources in order to create the largest and best studied statistical sample of radio-loud gravitationally lensed systems. With this survey, combined with detailed studies of the lenses found therein, powerful constraints can be placed on the cosmography (ie. expansion rate, mean density, and cosmological constant) of the Universe. CLASS is aimed at identifying lenses where multiple images are formed from compact flat-spectrum radio sources. The lens configurations should be easily identifiable in the radio maps. Thus, CLASS is most efficient at finding galaxy-mass lenses (which will dominate the counts for surveys not targeted at clusters) with separations of around a few arcseconds.

One of the most important uses of galaxy-mass lenses is in the determination of cosmological parameters. In particular, time delays measured between the components in multiple image systems can directly determine the angular-diameter distances to the lens and lensed objects, and therefore the Hubble constant H_0. Because the lensing equations depend on a ratio of distances, they are not as sensitive to the deceleration parameter q_0 and cosmological constant, although if accurate delays were found to a number of lenses at a range of redshifts, some measurement of these quantities could be made. Of course, the success of the method depends upon the presence of measurable variability in the lensed source, and the ability to construct a well-constrained mass model for the lens, and thus only a few lenses found in CLASS will be ``golden lenses'' suitable as cosmological standards. The CLASS group is monitoring the quadruple lens 1608+656 found in the first year of the survey. In addition to specific lenses, the survey statistics as a whole can provide constraints on cosmological models.

The telescopes of VLA as seen during the CLASS-2 observing sessions in August 1995 (©1995 S.Myers).

The Very Large Array (VLA) is being used as the primary instrument for the CLASS survey. In its largest ``A'' configuration, the VLA provides high-quality images with angular resolution of 0.2 arc-seconds at a typical observing frequency of 8.5 GHz. In the first phase of CLASS, observed with the VLA in February - May 1994, 3258 radio sources were mapped. From among these, We have identified around 100 sources with extended structure or multiple components for follow-up; based upon lensing statistics from previous surveys, we expect to find around 6 lenses among these. So far, there have been four confirmed lenses identified from the CLASS-1 survey, including 1600+434, a 1.4" double (Jackson etal. 1995), and 1608+656, a 2.1 quadruple image lens (Myers etal. 1995). The observations for the second phase, CLASS-2, were carried out in August and September 1995. In these sessions, an additional 4465 targets were imaged, bringing the CLASS total to 7723 sources. Preliminary examination of the CLASS-2 images and prompt follow-up found one new quadruple image lens system, and a large number of multiple-component candidates to study.

There have been previous VLA-based lens surveys: the MIT-Green Bank (MG) survey, and the Jodrell Bank VLA astrometric source survey (JVAS). In the ~5000 target sources observed in these surveys, a number of multiply imaged lens systems have been found (0218+357, 0414+0534, 1422+231, 1938+666).

The success of the CLASS survey, and the unprecedented observing and analysis speed, was made possible through automation of the observation scheduling and mapping analysis pipeline. Specialized ``auto-mapping'' software, called DIFMAP, with scripting capability, was developed to carry out these tasks (Shepherd etal. 1994). This has allowed us to observe an average of one target source per minute with the VLA, and then later to map the calibrated data at the rate of better than one field per two minutes of IBM RS6000 or Sparc-20 workstation time.

The distribution of separations and redshifts of lenses found in surveys such as CLASS provide powerful constraints upon the cosmology, particularly the cosmological constant. A well-defined sample selection is necessary to understand the statistics of the lensing, which is in turn necessary to constrain the cosmology (e.g. Turner, Ostriker & Gott 1993, Kochanek etal. 1993). Multiply-imaged lenses with compact radio components are excellent targets for cosmological studies, for the reasons outlined above. However, interpretation of the statistical results from any observational survey in the context of theoretical models requires the control of selection effects in the data sample. In the case of CLASS, this means studying the effects of the lensing on the parent sample of the survey, the resulting image geometries, the ability of the auto-mapping to identify the lensed images, and possible loss of lenses from the survey.

In addition to the lens candidates and lensing-related applications, the CLASS survey provides a solid body of radio information for all the objects. Astrometric (50 mas) positions for the sample are determined, which refines the MERLIN/VLBA phase-reference source network to a spacing of < 1 degree.

Lenses Found in JVAS and CLASS (or part of sample):

Note: out of date - needs updating

Lens Sep & Morph z lens z src Lens ID1 Survey
B0218+357 0.33" double + ring 0.68 0.96 S JVAS
MG0414+054 2.09" quad 0.96 2,64 E JVAS & MG  
B1030+074 1.56" double 0.60 1.54 S JVAS
B1422+231 1.28" quad 0.34 3.62   JVAS
B1938+666 0.93" quad + ring   0.878     JVAS
CLASS B0128+437 0.542" quad       CLASS-1
CLASS B0739+366 0.54" double       CLASS-2
CLASS B0712+472 1.27" quad 0.41 1.34 S CLASS-1
CLASS B0827+525 2.82" double   2.064 not a lens? CLASS-2
CLASS B1127+385 0.70" double     S + S? CLASS-2
CLASS B1152+199 1.56" double 0.44 1.02   CLASS-3
CLASS B1359+154   1.65" quad   3.24 group CLASS-3
CLASS B1555+375 0.43" quad       CLASS-2
CLASS B1600+434 1.39" double 0.42 1.59 S CLASS-1
CLASS B1608+656 2.08" quad 0.64 1.39 S + S? CLASS-1
CLASS B1933+503 1.17" quad + quad + double   0.76     CLASS-1
CLASS B2045+265 1.86" quad 0.87 1.28   CLASS-2
CLASS B2319+051 1.36" double 0.62, 0.59   E + E CLASS-2

Notes: 1 Lens IDs E=clearly elliptical or early type galaxy, S = spiral, lenticular, or elongated early type galaxy, group = small group (more than two contributing members).

Other CLASS/JVAS publications:

AuthorsTitle/ADS linkReference/astro-ph
Fassnacht, C.D., et al. (2005) Mass Along the Line of Sight to the Gravitational Lens B1608+656: Galaxy Groups and Implications for H_0 ApJ, submitted; astro-ph/0510728
York, T., et al. (2005) The Hubble constant from the gravitational lens CLASS B0218+357 using the Advanced Camera for Surveys MNRAS, 357, 124-134 (2005); astro-ph/0405115
Biggs, A.D., et al. (2004) Radio, optical and infrared observations of CLASS B0128+437 MNRAS, 350, 949-961 (2004); stro-ph/0402128
McKean, J.P., et al. (2004) Keck spectroscopy of Cosmic Lens All-Sky Survey gravitational lenses MNRAS, 350, 167-174 (2004); astro-ph/0402116
Wucknitz, O., Biggs, A.D., Browne, I.W.A. (2004) Models for the lens and source of B0218+357: a LENSCLEAN approach to determine H0 MNRAS, 349, 14-30 (2004); astro-ph/0312263
Koopmans, L.V.E., et al. (2003) Extrinsic Radio Variability of JVAS/CLASS Gravitational Lenses MNRAS, 595, 712-718 (2003); astro-ph/0302189
Biggs, A.D., et al. (2003) Global 8.4-GHz VLBI observations of JVAS B0218+357 MNRAS, 338, 599-608 (2003); astro-ph/0209182
Phillips, P.M., et al. (2001) The JVAS/CLASS search for 6-arcsec to 15-arcsec image separation lensing MNRAS, 328, 1001-1015 (2001)
Marchã, M. J., et al. (2001) The CLASS blazar survey - I. Selection criteria and radio properties MNRAS, 326, 1455-1466 (2001); astro-ph/0108468
Augusto, P., et al. (2001) B2114+022: a distant radio source gravitationally lensed by a starburst galaxy MNRAS, 326, 1007-1014 (2001)
Biggs, A.D., Browne, I.W.A., Wilkinson, P.N. (2001) Intrinsic intraday variability in the gravitational lens system B0218+357 MNRAS, 323, 995-998 (2001); astro-ph/0012476
Biggs, A.D., et al. (2001) MERLIN/VLA imaging of the gravitational lens system B0218+357 MNRAS, 322, 821-826 (2001); astro-ph/0011142
Phillips, P.M., Browne, I.W.A., Wilkinson, P.N. (2001) ARCS, the Arcminute Radio Cluster-lens Search - I. Selection criteria and initial results MNRAS, 321, 187-198 (2001); astro-ph/0009366
Wilkinson, P.N., et al. (2001) Limits on the Cosmological Abundance of Supermassive Compact Objects from a Search for Multiple Imaging in Compact Radio Sources PRL, 86, 584-587 (2001); astro-ph/0101328

Older CLASS publications:

Old News:

The JVAS gravitational lens B1938+666 (©1995 N.Jackson).

CONTACT! What the @#!!$%&! is it? Sounds like Metallica ... (©1998 CLASS).

Steven T. Myers