The statistics of wide-separation lensed quasars

Abstract

ABSTRACT The absence of any wide-separation gravitational lenses in the Large Bright QuasarSurvey is used to place limits on the population of cluster-sized halos in the universe,and hence constrain a number of cosmological parameters. The results agree withprevious investigations in strongly ruling out the standard cold dark matter modelbut they are consistent with low-density universes in which the primordial fluctuationspectrum matches both cluster abundances and cosmic microwave background mea-surements. These conclusions are essentially independent of the cosmological constant,which is in stark contrast to the statistics of galaxy lenses. The constraints presentedhere are nullified if clusters have core radii of >∼ 10 kpc, but are free of a number ofpotential systematic errors, due to the homogeneity of the data.Key words: gravitational lensing – cosmology: theory – galaxies: clusters. 1 INTRODUCTIONThe fraction of quasars which are gravitationally-lensed byintervening mass concentrations is a function of the deflectorpopulation and the underlying cosmological model, whichcan then be constrained from the observed lensing frequency(e.g. Kochanek 1993, 1996). In most cases of quasar lensingknown to date the principal deflectors are elliptical galaxies(e.g. Keeton, Kochanek & Seljak 1997), and the number ofsuch lenses, together with the distribution of image separa-tions and deflector redshifts, has been used to place quitestringent limits on the galaxy population (e.g. Kochanek1993) and the cosmological model (e.g. Kochanek 1996;Falco, Kochanek & Mun˜oz 1998). Unfortunately, the depen-dencies of the expected lensing probability on the galaxypopulation and the cosmology cannot be easily separated.For instance, cosmological constant-dominated models over-predict the number of lenses if the local and cosmologicalgalaxy populations are similar, but can be made consis-tent with observations if high-redshift galaxies are opticallythick due to dust (e.g. Kochanek 1996; Malhotra, Rhoads &Turner 1997).One way to bypass the lack of knowledge about thehigh-redshift galaxy population is to concentrate on lenseswith larger image separations (i.e. >∼ 3 arcsec). These canonly be produced by groups and clusters of galaxies, a pop-ulation which is more closely linked to the underlying cos-