Helbig, P

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Abstract: I give a brief overview of the current status of some aspects of the Cosmic Lens All-Sky Survey (CLASS): description of the survey, current list of lens systems, cosmological parameters from lensing statistics, H_0 from time delays, dark lenses, wide-separation lenses

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   University of GroningenThe Current Status of CLASSHelbig, PPublished in:BiometrikaIMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite fromit. Please check the document version below.Document VersionPublisher's PDF, also known as Version of recordPublication date:2000Link to publication in University of Groningen/UMCG research databaseCitation for published version (APA):Helbig, P. (2000). The Current Status of CLASS. Biometrika, 87(1).CopyrightOther than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of theauthor(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons).The publication may also be distributed here under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license.More information can be found on the University of Groningen website: https://www.rug.nl/library/open-access/self-archiving-pure/taverne-amendment.Take-down policyIf you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediatelyand investigate your claim.Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons thenumber of authors shown on this cover page is limited to 10 maximum.Download date: 08-06-2022THE CURRENT STATUS OF CLASSP. HELBIG (for the CLASS collaboration)Rijksuniversiteit Groningen, Kapteyn Instituut, Postbus 800,NL-9700 AV Groningen, The NetherlandsI give a brief overview of the current status of some aspects of the Cosmic Lens All-Sky Survey,CLASS.1 Who and What?CLASS, the Cosmic Lens All-Sky Survey, is a collaboration between groups at the Jodrell BankObservatory (UK), ASTRON (NL), the Kapteyn Astronomical Institute at the University ofGroningen (NL), Caltech (USA), the University of Pennsylvania (USA) and NRAO (USA). Itis a survey for flat-spectrum radio sources (α ≥ −0.5 for Sf ∼ fα between L-band (Condon etal.1 (NVSS)) and C-band (Gregory et al.2 (GB6))) with the selection criteria 0 ≤ δ ≤ 75◦ (dueto the area covered by GB6), |b| > 10◦ and SC−band > 30 mJy: 11685 objects. The objectsthus selected were then observed with the VLA in A-configuration at X-band. CLASS hasmany goals, many of which are lens-related (measuring λ0 and Ω0 with lensing statistics andH0 with time delays, studying the properties of lensing galaxies). (The definition of CLASS hasevolved, mainly due to improvements in the input catalogues used. The above definition is thecurrent, final one. However, objects were observed which are not part of the currently definied‘statistically complete CLASS’ and, if they happened to be lens systems, were of course followedup.)2 Why and How?Radio surveys can be particularly good gravitational-lens surveys, for a number of reasons:• Using interferometry, the beam size is much smaller than the image separation.• Flat-spectrum objects are compact (i.e. (almost) point sources), allowing typical lensingmorphologies to be recognised easily.• Somewhat related to the previous point, the lensing probability depends on the crosssection determined by the lens population; for extended sources, the source geometrypartially determines what is recognised as a lens system.• Most sources are quasars at high redshift, which leads to a high lensing rate.• Since flat-spectrum objects are compact, they can be variable on relatively short timescales,which aids in determining time delays.• There is no bias from lens galaxies due to extinction by the lens or comparable brightnessof source and lens, as can be the case with optical surveys.• High-resolution followup is possible with interferometers such as MERLIN, the VLBA,VLBI. . . .But there is one disadvantage:• Additional work is required to get redshifts.From our own CLASS VLA observations (i.e. A-configuration at X-band), objects withmultiple compact (< 170 mas) are marked as lens candidates if the separation is between 300 masand 6 arcsec, the flux ratio <10:1 and the total flux at X-band in the compact componentsis >20mJy. These constraints are chosen so that sufficient followup is possible for all suchcandidates. Candidates are then observed at progressively higher resolution, first at C-band withMERLIN (50 mas resolution) and then, if necessary, the VLBA (3 mas resolution) and finally ifneeded with VLBI, at additional frequencies if needed. About 80% of the candidates are rejectedafter the MERLIN ‘filter’ due to different surface brightnesses in the components (gravitationallensing, of course, conserves surface brightness). Other reasons for rejection include obviousnon-lens structure (such as a core-jet morphology) and different spectral indices or polarisation.It is interesting to note that, with one possible exception, every candidate which has passed thesetests and thus been deemed to be a lens system on the basis of radio data alone has yielded alens galaxy when observed optically (usually with HST). Since the source and lens redshifts areneeded for various purposes, these are also checked to be consistent with lensing (zl < zs, thesame zs for all components of the source), though in practice no candidates which have passedthe above-mentioned tests have been ruled out on this basis.Table 1 shows the current CLASS and JVAS lens systems. It should be noted that missingredshifts are mostly from relatively new systems (though in the case of B1938+666, a redshiftmight could have been measured if UKIRT were able look this far north). It thus appears tobe a realistic goal to have the survey complete with respect to both source and lens redshiftswithin the next several months. Note also the relatively narrow range of image separations,which is definitively not due to any sort of bias or incompleteness but reflects a real fact aboutthe universe (more precisely, the mass spectrum of lens galaxies).3 λ0 and Ω0 from Lensing StatisticsWe have done an analysis of JVAS (essentially the brightest 2308 sources in CLASS). Lensingstatistics, at least in the interesting part of parameter space, essentially measures λ0 − Ω0. Asdiscussed in Helbig3, we obtain−2.69 < λ0 −Ω0 < +0.68 (1)at 95% confidence; for a flat universe, this corresponds to−0.85 < λ0 −Ω0 < +0.84 (2)Table 1: The JVAS and CLASS gravitational lensesSurvey Name # images ∆θ′′ zl zs lens galaxyconfirmed lensesCLASS B0128+437 4 0.542 ? ? ?JVAS B0218+357 2 + ring 0.334 0.6847 0.96 spiralJVAS MG0414+054 4 2.09 0.9584 2.639 ellipticalCLASS B0712+472 4 1.27 0.406 1.34 spiralCLASS B0739+366 2 0.540 ? ? ?JVAS B1030+074 2 1.56 0.599 1.535 spiralCLASS B1127+385 2 0.701 ? ? ?CLASS B1152+119 2 1.56 0.439 1.019 ?CLASS B1359+154 4 1.65 ? 3.212 ?JVAS B1422+231 4 1.28 0.337 3.62 ?CLASS B1555+375 4 0.43 ? ? ?CLASS B1600+434 2 1.39 0.414 1.589 spiralCLASS B1608+656 4 2.08 0.63 1.39 spiralCLASS B1933+507 4 + 4 + 2 1.17 0.755 2.62 ?JVAS B1938+666 4 + 2 0.93 0.878 ? ?CLASS B2045+265 4 1.86 0.867 1.28 ?CLASS B2319+051 2 1.365 0.624 ? ?puzzling probable lensesJVAS B2114+022 2 or 4 2.57 0.32 & 0.59 ? ?It should be noted that the probability distribution in the λ0-Ω0 plane is not Gaussian; inparticular, the above numbers were obtained from ‘real contours’ (cf. Helbig4 and referencestherein) and not by plotting contours at some fraction of the peak likelihood, and of course theydepend on the region of parameter space examined as long as there is a non-negligible likelihoodoutside of it. These caveats should be kept in mind when comparing these constraints to othersin the literature.Of course, not only an analysis of CLASS but a better analysis of CLASS is in the works.However, first the survey and lens followup have to be finished and the S-z plane of the parentpopulation (i.e. non-lenses in the survey and objects ‘amplified in’ to the survey by lensing) mustbe determined. This information is needed in two quite distinct contexts, even though they arereally two sides of the same coin:• The number–flux-density relation at the redshifts of the sources in the lens systems isneeded for the calculation of the amplification bias.• The flux-density–dependent redshift distribution is needed as a proxy for the redshifts ofthe non-lenses in the survey.Even without this information, however, there is the interesting possibility of using CLASS forthe lens-redshift test; see Helbig5 for discussion.4 H0 from Time DelaysSince all observables in a gravitational lens system are dimensionless except for the time delaybetween the images, this can be used to scale the model of the system and thus, if the redshiftsare known, measure H0 (with a higher-order dependency on λ0 and Ω0). A radio survey offersthe advantages that microlensing is less of a worry and that the sources are ‘pre-selected’ tobe variable (due to their flat-spectrum nature, as mentioned above). The angular separationprobed by CLASS corresponds to galaxy-mass lenses and thus time delays of weeks, which isconvenient. To date, there are 6 measured gravitational-lens time delays, 3 from CLASS, ofwhich 5 agree quite well (see, e.g., Koopmans & Fassnacht6 for discussion). In principle, as firstpointed out by Refsdal7, one can use time delays from several systems to measure λ0 and Ω0.Although it is too early to make a definitive statement, it is interesting to note that the derivedvalues for H0 agree better if currently favoured values for λ0 and Ω0 are assumed. To quote anumber for posterity, H0 = 68. More CLASS systems are being monitored, so statistics shouldimprove in the future.5 Dark Lenses?As mentioned above, there is only one possible lens system which has passed all the radio testsbut in which no lens galaxy has been detected, and could thus be a ‘dark lens’ (cf. Jackson etal.8 and references therein). However, statistical arguments favour the alternative explanation ofthis system, B0827+525, being the first binary radio-loud QSO. On the other hand, the case isnot clear-cut. If the latter explanation is true, it will have the smallest separation of all knownQSO pairs. On the other hand, if it is a lens system, it will have the largest separation of allCLASS lenses. See Koopmans et al.9 for further discussion.6 Wide-Separation LensesAlthough CLASS originally examined the range between 300 mas and 6 arcsec, this has beenextended up to an arc-minute in two new surveys. First, CLASS has been extended to searchup to 15 arcsec in a manner identical to the original survey, made possible by the increase incomputing power since CLASS began. Second, a new survey, the Arc-Minute Radio Cluster-Lens Search (ARCS), has looked for lensing of extended sources on the 15 to 60 arcsec scale(due to the longer time delays, many of the arguments used to rule out candidates at smallerseparation will not work at larger separation and the number of chance coincidences on the skyof course increases with the separation, so a different strategy is called for). To date, no lenssystems have been found in this range of angular separation, though one candidate remains.More details can be found in Phillips et al.10.AcknowledgmentsIt is a pleasure to thank the rest of the CLASS collaboration for input.References1. J.J. Condon et al., AJ 115, 1693 (1998)2. P.C. Gregory et al.,ApJS 103, 427 (1996)3. P. Helbig et al., A&A 136, 297 (1999)4. P. Helbig A&A 350, 1 (1999)5. P. Helbig, these proceedings6. L.V.E. Koopmans & C.D. Fassnacht, ApJ 527, 513 (1999)7. S. Refsdal, MNRAS 132, 101 (1966)8. N. Jackson et al., A&A 334, L33 (1998)9. L. Koopmans et al., A&A, in press10. P. Phillips et al., MNRAS, submitted

The Current Status of CLASS

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The Current Status of CLASS

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