The 2 degree Field (2dF) galaxy redshift survey will involve obtaining
approximately 2.5 x 10^5 spectra of objects previously identified as galaxy
candidates on morphological grounds. Included in these spectra should be about
ten gravitationally-lensed quasars, all with low-redshift galaxies as
deflectors (as the more common lenses with high-redshift deflectors will be
rejected from the survey as multiple point-sources). The lenses will appear as
superpositions of galaxy and quasar spectra, and both cross-correlation
techniques and principal components analysis should be able to identify
candidates systematically. With the 2dF survey approximately half-completed it
is now viable to begin a systematic search for these spectroscopic lenses, and
the first steps of this project are described here.Comment: PASA (OzLens edition), in press; 4 pages, 0 figure

Cole

Daniel J. Mortlock

Darren S. Madgwick

Hewett

Ofer Lahav

Warren

English

arXiv

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The 2dF Gravitational Lens Survey

The 2 degree Field (2dF) galaxy redshift survey will involve obtaining spectra of approximately 2.5 105 objects which have previously been identified as galaxy candidates on morphological grounds. Included in these spectra should be about ten gravitationally-lensed quasars, all with low-redshift galaxies as deflectors (as the more common lenses with high-redshift deflectors will be rejected from the survey as multiple point-sources). The lenses will appear as superpositions of galaxy and quasar spectra, and either cross-correlation techniques or principal components analysis should be able to identify candidates systematically. With the 2dF survey approximately half-completed it is now viable to begin a methodical search for these spectroscopic lenses, and the first steps of this project are described here

Mortlock, DJ

Madgwick, DS

Lahav, O

Spiral - Imperial College Digital Repository

arXiv:astro-ph/0103207v1  14 Mar 2001The 2dF gravitational lens surveyDaniel J. Mortlock1,2 Darren S. Madgwick1 Ofer Lahav11 Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, United Kingdommortlock@ast.cam.ac.uk, dsm@ast.cam.ac.uk, lahav@ast.cam.ac.uk2 Astrophysics Group, Cavendish Laboratory, Madingley Road, Cambridge CB3 0HE, UnitedKingdomAbstractThe 2 degree Field (2dF) galaxy redshift survey will involve obtaining spectra of ap-proximately 2.5× 105 objects which have previously been identified as galaxy candidates onmorphological grounds. Included in these spectra should be about ten gravitationally-lensedquasars, all with low-redshift galaxies as deflectors (as the more common lenses with high-redshift deflectors will be rejected from the survey as multiple point-sources). The lenseswill appear as superpositions of galaxy and quasar spectra, and either cross-correlation tech-niques or principal components analysis should be able to identify candidates systematically.With the 2dF survey approximately half-completed it is now viable to begin a methodicalsearch for these spectroscopic lenses, and the first steps of this project are described here.Keywords: surveys – gravitational lensing – methods: data analysis1 IntroductionIt is possible to discover gravitational lenses spectroscopically by searching for quasar emission linesin the spectra of local galaxies. This is the only way to find lenses with low-redshift deflectors; theseare particularly valuable as the lens galaxies can be studied in great detail and the timescales formicrolensing are short enough to permit a range of interesting measurements (Mortlock & Webster2000 and references therein). Kochanek (1992) and Mortlock & Webster (2000) calculated theexpected event rate, and several tens of spectroscopic lenses should be forthcoming from thecurrent generation of galaxy redshift surveys (GRSs).1The first large GRS, the Center for Astrophysics (CfA) survey (Geller & Huchra 1989), resultedin the discovery of the lens Q 2237+0305 (Huchra et al. 1985), although it was an essentiallyserendipitous event. There have been no other quasar lenses discovered in this manner, but severalsearches for lensed emission line galaxies have been undertaken. The only confirmed spectroscopicdiscoveries were the two Einstein rings found by Warren et al. (1996) and Hewett et al. (1999) intheir sample of large elliptical galaxies. Even though there are only several hundred galaxies in thesurvey, the regularity of their spectra is such that a very sensitive search for discrepant emissionlines is possible (Willis et al. 2001); several more candidates have been identified and are awaiting1The use of existing spectra is critical to the efficiency of this method; this is discussed elsewhere in this issueby Mortlock & Drinkwater (2001).1re-observation. Hall et al. (2000) also announced the identification of several candidate lenses inthe Canadian Network for Observational Cosmology (CNOC) GRS (Yee et al. 2000), althoughthese too are yet to be confirmed. The next generation of galaxy survey is exemplified by theSloan Digital Sky Survey (SDSS; York et al. 2000) which, with ∼ 106 galaxy spectra, should yieldat least 50 spectroscopic lenses (not to mention the several hundred lenses expected to be resolvedin the imaging survey). The SDSS has only recently got under way, but, at the time of writing,over 5 × 104 spectra have been obtained, and the pipeline reduction software should be able toselect lens candidates with some reliability, although none have been announced so far.This paper details the beginnings of a lens survey based on the spectra taken as part of thelargest existing galaxy sample, the 2 degree Field (2dF) GRS (Section 2). After a brief discussionof the lens statistics (Section 3), the possible spectral analysis methods are examined (Section 4).These points are summarised in Section 5.2 The 2dF galaxy redshift surveyThe 2dF GRS (e.g., Folkes et al. 1999; Cole et al. 2001) is, formally, a spectroscopic surveyof ∼ 2.5 × 105 extended images, all of which have isophotal magnitudes brighter than BJ =19.45. The data are obtained using the 2dF instrument, an automatically-configurable multi-fibrespectrograph on the Anglo-Australian Telescope (AAT). The ∼ 105 spectra obtained so far have aresolution of about 10 Å, and cover the range from ∼ 3700 Å to ∼ 8100 Å. Whilst the wavelengthcalibration is excellent, there have been some problems with the flux calibration, and so continuumlevels remain quite uncertain. The spectra are typically the result of 45-min integrations, and sothe signal-to-noise ratio in each pixel is between ∼ 10 and ∼ 20 for an object at the survey limit.It is expected that ∼ 95 per cent of the objects selected for the survey will be local galaxies,with the remainder being mainly misidentified Galactic stars. However a few of the survey galaxieswill be multiply-imaging background quasars, and, if the quasars are sufficiently bright, shouldbe identifiable as gravitational lenses. Two main questions present themselves at this point: howmany lenses might be detectable in these spectra, and what is the best way to find any lensesthat might be present? The answers are related, in so far as any given search method is linked tothe effective “depth” of the lens survey, but it is possible to obtain an estimate of the number oflenses without fully understanding how to find them.3 Lens statisticsKochanek (1992) and Mortlock & Webster (2000) applied standard techniques to determine therate of lensed quasars expected in a generic GRS, and a more detailed calculation verified theseresults for the 2dF survey (Mortlock & Webster 2001). The number of lenses depends principallyon two factors: the depth of the survey and the quality of the spectra. The latter, potentially asomewhat ambiguous notion, can be characterised in terms of ∆mqg, which is defined as follows(Kochanek 1992): for a lens in which the galaxy has magnitude mg and the quasar images havetotal magnitude mq, the presence of the quasar emission lines is detectable if mq ≤ mg +∆mqg,but the quasar is undetectable if it is fainter than this. The value of ∆mqg clearly increases withthe integration time of the observations, but is also critically dependent on the properties of galaxyand quasar spectra. Kochanek (1992) estimated ∆mqg ≃ 4 for the SDSS spectra, and Mortlock& Webster (2000) used ∆mqg ≃ 2 for the lower quality 2dF spectra. These figures then led tothe estimate that the 2dF GRS should yield about 10 lensed quasars (Mortlock & Webster 2000),although it could be as low as ∼ 5 (if ∆mqg ≃ 1) or as high as ∼ 20 (if ∆mqg ≃ 3). Fortunately the2value of ∆mqg appropriate to a given search technique can be evaluated by analysing simulatedcomposite lens spectra.4 The 2dF gravitational lens surveyThe process of finding lensed quasars in the 2dF galaxy spectra can be split into three quite distinctphases. The first of these is the identification of candidate lenses on purely spectroscopic grounds.The vast majority of the spectra should be uninteresting, at least in the present application, buta small fraction should stand out as having either unusual continua, or, more likely, what may bebroad emission lines. This set of (probably) several thousand spectra will include white dwarfs;unlensed quasars misidentified by the APM software; and, with luck, several lensed quasars. Thesecond phase of the analysis is to examine each spectrum in detail (possibly by eye) with a view toremoving all those objects which are clearly not quasars. This step could also profitably includeusing any other data which is already available, such as images from the Digitized Sky Survey orthe Schmidt plates from which the 2dF objects were selected. Finally, the (hopefully) small setof strong lens candidates would have to be imaged at high resolution (< 1 arcsec) with a view todetermining their nature morphologically. In terms of quantifiable resources this last step is by farthe most expensive, as the automatic spectral analysis should take less than a second per object.However the opposite is true when intellectual effort is considered, as the manual inspection of theinitial candidate list and (especially) the imaging observations are conceptually straight-forward.The complex task is determining how best to find a subset of the survey spectra that contains allthe lenses and not too much else.One approach to this problem would be to work backwards – if it is assumed that all thedetectable lenses will primarily have broad quasar emission lines then the task reduces to that ofidentifying the spectra that have emission line-like features. Cross-correlation techniques, com-bined with continuum removal, could certainly detect some such objects. Unfortunately manygalaxy spectra also have features that could be confused with the broad lines of a fainter, su-perimposed spectrum. Further, this is clearly an inefficient procedure, as much of the availableinformation remains unused. Finally, such a method becomes increasingly difficult once any at-tempt is made to include non-Gaussian effects such as variation in the continuum levels and thepresence of sky lines.The method that has been adopted is based on Monte Carlo simulations of a large numberof lenses. Using the formalism described in Mortlock & Webster (2000) it is possible to gener-ate deflector-source pairs that reflect the variation in lensing likelihood with source redshift anddeflector properties. A galaxy-quasar pair generated in this way is a composite object with onlyone degree of freedom: the relative contribution of the two components to the resultant spectrum.This is very sensitive to the properties of the lens, but such details need not be treated explicitlyas the range of possible weightings can be analysed without knowing the actual image configura-tions. The most important aspect of this method is that, for a given galaxy-quasar pair, a veryrealistic composite spectrum can be generated by simply combining real 2dF galaxy and quasarspectra. The noise properties of the spectra are certainly correct provided both spectra are kept intheir observed frame; the one possible important omission is reddening of the background quasarby dust in the lens. This too can be included in the simulations, but it would incur the penaltyof parameterisation; this process is otherwise model-independent. A promising candidate for theanalysis method is principal components analysis (PCA; Murtagh & Hecht 1987), which is alreadybeing used extensively within the 2dF collaboration, and is well suited to the task at hand. Thesurvey galaxies cover a definite region in the multi-dimensional space of component coefficients3(e.g., Folkes et al. 1999) and the lenses should inhabit a contiguous region, with the distance fromthe galactic locus increasing with the brightness of the quasar component. Unfortunately this pro-cess has not yet been implemented; the central ideas are quite clear, but quantitative estimates ofthe completeness and efficiency of the lens survey cannot yet be made.5 ConclusionsIt should be possible to find ∼ 10 gravitationally-lensed quasars in the 2dF GRS spectra. Cross-correlation techniques can be used to search for quasars’ emission lines, but a more powerful (andmore simply implemented) scheme is to generate large numbers of simulated lens spectra andto then analyse them using existing software. If the quasar and galaxy spectra used are takenfrom the 2dF samples this method has the enormous advantage of automatically including varioussubtleties of the survey data, without the need for detailed modelling. By identifying the regionsof parameter space inhabited by these spectra, it is possible to characterise the sensitivity of thelens search (effectively measuring the ∆mqg parameter discussed in Section 3). More importantly,the real spectra that fall into the same regions of parameter space are immediately identifiable ascandidate lenses. Implementation of these techniques should bring the first positive results withinthe next year.AcknowledgementsThanks to Rachel Webster for first suggesting this avenue of investigation, and also to the entire2dF survey team; their support will be critical to the success of this project. DJM is funded byPPARC.ReferencesCole S., et al., 2001, MNRAS, submitted (astro-ph/0012429)Folkes, S.R., et al., 1999, MNRAS, 308, 459Geller, M.J., Huchra, J.P., 1989, Science, 246, 897Hall, P.B., et al., 2000, AJ, 120, 1660Hewett, P.C., Warren, S.J., Willis, J.P., Bland-Hawthorn, J., Lewis, G.F., 1999, in Imaging theUniverse in Three Dimensions, eds. van Breugel, W., Bland-Hawthorn, J., ASP, 94Huchra, J.P., Gorenstien, M., Kent, S., Shapiro, I., Smith, G., Horine, E., Perley, R., 1985, AJ,90, 691Kochanek, C.S., 1992, ApJ, 397, 381Mortlock, D.J., Drinkwater, M.J., 2001, PASA, in pressMortlock, D.J., Webster, R.L., 2000, MNRAS, 319, 879Mortlock, D.J., Webster, R.L., 2001, MNRAS, 321, 629Murtagh, F., Hecht, A., 1987, Multivariate Data Analysis, ReidelWarren, S.J., Hewett, P.C., Lewis, G.F., Møller, P., Iovino, A., Shaver, P.A., 1996b, MNRAS,278, 139Willis, J.P., Hewett, P.C., Warren, S.J., Lewis, G.F., 2001, MNRAS, submittedYee, H.K.C., et al., 2000, ApJS, 129, 475York, D.G., et al., 2000, AJ, 120, 15794

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The 2dF gravitational lens survey

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