Publications
221 results found
Mortlock DJ, Madgwick DS, Lahav O, 2001, The 2dF gravitational lens survey, Publications of the Astronomical Society of Australia, Vol: 18, Pages: 192-194, ISSN: 1323-3580
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, Drinkwater MJ, 2001, Using the 6dF galaxy redshift survey to detect gravitationally-lensed quasars, Publications of the Astronomical Society of Australia, Vol: 18, Pages: 195-197, ISSN: 1323-3580
It is possible to detect gravitationally-lensed quasars spectroscopically if the spectra obtained during galaxy surveys are searched for the presence of quasar emission lines. The up-coming 6 degree Field (6dF) redshift survey on the United Kingdom Schmidt Telescope will involve obtaining ~105 spectra of near-infrared selected galaxies to a magnitude limit of K = 13. Applying previously developed techniques implies that at least one lens should be discovered in the 6dF survey, but that as many as ten could be found if quasars typically have B J – K ~ 8. In this model there could be up to fifty lensed quasars in the sample, but most of them could only be detected by infrared spectroscopy.
Mortlock DJ, Webster RL, 2001, Using the 2 degree field galaxy redshift survey to detect gravitationally lensed quasars, Monthly Notices of the Royal Astronomical Society, Vol: 321, Pages: 629-641, ISSN: 0035-8711
Galaxy redshift surveys can be used to detect gravitationally lensed quasars if the spectraobtained are searched for the emission lines of quasars. Previous investigations of thispossibility have used simple models to show that the 2 degree Field (2dF) redshift surveycould yield several tens of new lenses, and that the larger Sloan Digital Sky Survey shouldcontain an order of magnitude more. However the particular selection effects of the sampleswere not included in these calculations, limiting the robustness of the predictions; thus amore detailed simulation of the 2dF survey was undertaken here. The use of an isophotalmagnitude limit reduces both the depth of the sample and the expected number of lenses, butmore important is the Automatic Plate Measuring survey's star±galaxy separation algorithm,used to generate the 2dF input catalogue. It is found that most quasar lenses are classed asmerged stars, with only the few lenses with low-redshift deflectors likely to be classified asgalaxies. Explicit inclusion of these selection effects implies that the 2dF survey shouldcontain 10 lenses on average. The largest remaining uncertainty is the lack of knowledge ofthe ease with which any underlying quasars can be extracted from the survey spectra.
Mortlock DJ, Webster RL, 2001, Gravitational lensing in galaxy redshift surveys, Gravitational Lensing: Recent Progress and Future Goals, Publisher: Astronomical Society of the Pacific, Pages: 47-48, ISSN: 1050-3390
Gravitationally-lensed quasars should be discovered as a by-product of largegalaxy redshift surveys, being discovered spectroscopically when a low-redshiftgalaxy exhibits high-redshift quasar emission lines. The number of lensesexpected is higher than previously estimated, mainly due to the fact that thepresence of the quasar images brings faint deflector galaxies above the surveylimit. Thus the a posteriori likelihood of the discovery of Q 2237+0305 in theCenter for Astrophysics redshift survey is approximately 0.03. In the future,the 2 degree Field survey should yield at least 10 lensed quasars, and theSloan Digitial Sky Survey up to 100.
Mortlock DJ, Webster RL, 2000, Using galaxy redshift surveys to detect gravitationally lensed quasars, Monthly Notices of the Royal Astronomical Society, Vol: 319, Pages: 879-892, ISSN: 0035-8711
Gravitationally lensed quasars can be discovered as a by-product of galaxy redshift surveys. Lenses discovered spectroscopically in this way should require less observational effort per event than those found in dedicated lens surveys. Further, the lens galaxies should be relatively nearby, facilitating a number of detailed observations that are impossible for the more common high-redshift lenses. This is epitomized by the wide range of results that have been obtained from Q 2237+0305, which was discovered as part of the Center for Astrophysics redshift survey, and remains the only quasar lens discovered in this way. The likelihood of this survey yielding a lens is calculated to be ∼0.03, which is an order of magnitude larger than previous estimates due to two effects. First, the quasar images themselves increase the observed flux of the lens, so that lens galaxies up to a magnitude fainter than the nominal survey limit must be included in the calculation. Secondly, it is possible for lensed quasars with extremely faint deflectors to enter the survey due to the extended morphology of the multiple images. Extrapolating these results to future surveys, the 2 degree Field galaxy redshift survey should contain between 10 and 50 lenses and the Sloan Digital Sky Survey should yield between 50 and 300 lenses, depending on the cosmological model and the observing conditions.
Mortlock DJ, Webster RL, 2000, Gravitational lensing by elliptical galaxies, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 319, Pages: 860-871, ISSN: 0035-8711
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- Citations: 7
Mortlock DJ, Webster RL, 2000, The statistics of wide-separation lensed quasars, Monthly Notices of the Royal Astronomical Society, Vol: 319, Pages: 872-878, ISSN: 0035-8711
The absence of any wide-separation gravitational lenses in the Large Bright Quasar Survey is used to place limits on the population of cluster-sized haloes in the universe, and hence constrain a number of cosmological parameters. The results agree with previous investigations in strongly ruling out the standard cold dark matter model but they are consistent with low-density universes in which the primordial fluctuation spectrum matches both cluster abundances and cosmic microwave background measurements. These conclusions are essentially independent of the cosmological constant, which is in stark contrast to the statistics of galaxy lenses. The constraints presented here are nullified if clusters have core radii of ≳10 kpc, but are free of a number of potential systematic errors, owing to the homogeneity of the data.
Mortlock DJ, Webster RL, 2000, Gravitational lensing by elliptical galaxies., Monthly Notices of the Royal Astronomical Society, Vol: 319, Pages: 860-871, ISSN: 0035-8711
The fraction of high-redshift sources which are multiply-imaged byintervening galaxies is strongly dependent on the cosmological constant, and so can be a useful probe of the cosmological model. However its power is limited by various systematic (and random) uncertainties in the calculation of lensing probabilities, one of the most important of which is the dynamical normalisation of elliptical galaxies. Assuming ellipticals' mass distributions can be modelled as isothermal spheres, the mass normalisation depends on: the velocity anisotropy; the luminosity density; the core radius; and the area over which the velocity dispersion is measured. The differences in the lensing probability and optical depth produced by using the correct normalisation can be comparable to the differences between even the most extreme cosmologicalmodels. The existing data is not sufficient to determine the correctnormalisation with enough certainty to allow lensing statistics to be used to their full potential. However, as the correct lensing probability is almostcertainly higher than is usually assumed, upper bounds on the cosmological constant are not weakened by these possibilities.
Wyithe JSB, Webster RL, Turner EL, et al., 2000, A gravitational microlensing determination of continuum source size in Q2237+0305, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 315, Pages: 62-68, ISSN: 0035-8711
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- Citations: 51
Mortlock DJ, Webster RL, Francis PJ, 1999, Binary quasars, Monthly Notices of the Royal Astronomical Society, Vol: 309, Pages: 836-846, ISSN: 0035-8711
Quasar pairs are either physically distinct binary quasars or the result of gravitational lensing. The majority of known pairs are in fact lenses, with a few confirmed as binaries, leaving a population of objects that have not yet been successfully classified. Building on the arguments of Kochanek, Falco & Muñoz, it is shown that there are no objective reasons to reject the binary interpretation for most of these. In particular, the similarity of the spectra of the quasar pairs appears to be an artefact of the generic nature of quasar spectra. The two ambiguous pairs discovered as part of the Large Bright Quasar Survey (Q 1429-053 and Q 2153-0256) are analysed using principal components analysis, which shows that their spectral similarities are not greater than expected for a randomly chosen pair of quasars from the survey. The assumption of the binary hypothesis allows the dynamics, time-scales and separation distribution of binary quasars to be investigated and constrained. The most plausible model is that the activity of the quasar is triggered by tidal interactions in a galatic merger, but that the (re-)activation of the galactic nuclei occurs quite late in the interaction, when the nuclei are within 80±30 kpc of each other. A simple dynamical friction model for the decaying orbits reproduces the observed distribution of projected separations, but the decay time inferred is comparable to a Hubble time. Hence it is predicted that binary quasars are only observable as such in the early stages of galactic collisions, after which the quiescent supermassive black holes orbit in the merger remnant for some time.
Fluke CJ, Webster RL, Mortlock DJ, 1999, The ray-bundle method for calculating weak magnification by gravitational lenses, MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Vol: 306, Pages: 567-574, ISSN: 0035-8711
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- Citations: 23
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