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The high-redshift radio-loud quasar PKS 2126−158 is found to have a large number of red galaxies in close apparent proximity. We use the Gemini Multi-Object Spectrograph (GMOS) on Gemini South to obtain optical spectra for a large fraction of these sources. We show that there is a group of galaxies at , coincident with a metal-line absorption system seen in the quasar's optical spectrum. The multiplexing capabilities of GMOS also allow us to measure redshifts of many foreground galaxies in the field surrounding the quasar. The galaxy group has five confirmed members, and a further four fainter galaxies are possibly associated. All confirmed members exhibit early-type galaxy spectra, a rare situation for a Mg ii absorbing system. We discuss the relationship of this group to the absorbing gas, and the possibility of gravitational lensing of the quasar due to the intervening galaxies. [ABSTRACT FROM AUTHOR]

Abstract We determine the mass distribution of a spiral galaxy, 2237+0305, using both gravitational lensing and dynamical constraints. We find that lensing can break the disc–halo degeneracy. 2237+0305 has a submaximal disc, contributing 57 ± 3 per cent of the rotational support at the disc maximum. The disc mass-to-light ratio is 1.1 ± 0.2 in the I band and the bulge, 2.9 ± 0.5 . The dark matter halo, modelled as a softened isothermal sphere, has a large core radius (13.4 ± 0.4 kpc ≡ 1.4r[sub d]) to high accuracy for the best-fitting solution. The image positions are reasonably well fitted, but require further rotation information to obtain a unique solution. [ABSTRACT FROM AUTHOR]

The fraction of high-redshift sources which are multiply imaged by intervening 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 normalization of elliptical galaxies. Assuming ellipticals’ mass distributions can be modelled as isothermal spheres, the mass normalization 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 normalization can be comparable to the differences between even the most extreme cosmological models. The existing data are not sufficient to determine the correct normalization with enough certainty to allow lensing statistics to be used to their full potential. However, as the correct lensing probability is almost certainly higher than is usually assumed, upper bounds on the cosmological constant are not weakened by these possibilities. [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]