An Optical Imaging Search for Small-Separation Gravitational Lenses

Details An Optical Imaging Search for Small-Separation Gravitational Lenses An Optical Imaging Search for Small-Separation Gravitational Lenses

Dec 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.4515s&link_type=abstract

American Astronomical Society, 181st AAS Meeting, #45.15; Bulletin of the American Astronomical Society, Vol. 24, p.1194

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We are conducting a search for gravitationally lensed quasars, using ground-based CCD imaging, and subsequent image processing. The sample of target quasars has been selected by the apparently high absolute luminosity (magnification bias) and a high redshift (large intercept length). For a brief description of the survey see, e.g., Djorgovski & Meylan 1989, IAU Symp. #134, p. 269. Some obvious lens candidates with image separations of a few arcsec have been discovered in this and other, similar surveys. Empirically, about 1 in 100 or 200 quasars selected in this way turns out to be a gravitational lens. Simple theoretical models, e.g., by Turner et al. 1984, ApJ 284, 1, predict that many more lenses should be found at smaller angular separations, with the distribution peaking at ~ 0.5 arcsec. In order to probe this regime, we are reanalysing the imaging data on some 300 quasars, using the PSF subtraction technique. Our simulations indicate that we should be able to detect equal-brightness pairs at separations as small as 0.3 arcsec or so; we were also able to ``rediscover'' the lensed quasar 1208+101 using our techniques. To date, we have reanalysed data on about 100 quasars, and more should be done by the time of the meeting. No new lens candidates have been identified so far. This is already in mild conflict with the theory, if we use the empirical normalization with ``wide'' pairs, as described above. The lack of small angular separation lenses is fully consistent with the results obtained by Bahcall, Maoz, and their collaborators in the HST snapshot survey, and by other groups who used ground-based imaging. Once the statistical analysis is complete, these results may provide useful constraints for theoretical models. As a byproduct of our survey, we have found a large number of faint galaxies near the lines of sight towards our target quasars; their follow-up spectroscopy may be useful for studies of absorption line systems.

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