QSO lensing magnification: a comparison of 2QZ and Sloan Digital Sky Survey results

Abstract

The lensing magnification of background QSOs by foreground galaxies and clusters is a powerful probe of the mass density of the Universe and the power spectrum of mass clustering. However, the observational results in this area have been controversial. In particular, the results of Myers et al. from the 2dF QSO survey suggested that a strong anticorrelation effect at g < 21 is seen for both galaxies and clusters which implied that galaxies are antibiased (b≈ 0.1) on small scales at a higher level than predicted by the standard cosmology whereas the results of Scranton et al. from the Sloan Digital Sky Survey (SDSS) suggested that the effect was much smaller (b≈ 1) and in line with standard expectations. We first cross-correlate the SDSS photo-z, g < 21, 1.0 < zp < 2.2 QSOs with g < 21 galaxies and clusters in the same areas. The anticorrelation found is somewhat less than the results of Myers et al. based on 2QZ QSOs. But contamination of the QSOs by low-redshift narrow emission-line galaxies and QSOs can cause underestimation of the anticorrelation lensing signal. Correcting for such low-redshift contamination at the levels indicated by our spectroscopic checks suggests that the effect is generally small for QSO cross-correlations with g < 21 galaxies but may be an issue for fainter galaxy samples. Thus when this correction is applied to the photo-z QSO sample of Scranton et al. the anticorrelation increases and the agreement with the 2QZ results of Myers et al. is improved. When we also take into account the fainter r < 21 galaxy limit of Scranton et al. as opposed to g < 21 for Myers et al., the two observational results appear to be in very good agreement. This therefore leaves open the question of why the theoretical interpretations are so different for these analyses. We note that the results of Guimaraes, Myers and Shanks based on mock catalogues from the Hubble Volume simulation strongly suggest that QSO lensing at the levels detected by both Myers et al. and now Scranton et al. is incompatible with a galaxy bias of b≈ 1 in the standard cosmological model. If the QSO lensing results are correct then the consequences for cosmology may be significant.