QSO-galaxy correlations: lensing or dust?
Scott M. Croom, PASA, 18 (2), in press.
Next Section: Introduction
Contents Page: Volume 18, Number 2

QSO-galaxy correlations: lensing or dust?

Scott M. Croom ¹

¹ Anglo-Australian Observatory, PO Box 296, Epping, NSW 2121, Australia

Abstract:

We present evidence for an anti-correlation between faint QSOs and B<23 galaxies. A sample of 192 QSOs in a 2.5deg² area has been imaged using the Issac Newton Telescope Wide-Field Camera. The cross-correlation signal is of a similar amplitude to the galaxy auto-correlation function at the limit of B<23, but is negative in sign. As fainter galaxies are selected the negative correlation signal becomes less significant, until the signal is effectively zero at B<26. We propose two alternate explanations for the observed effect. The first is gravitational lensing of the faint background QSOs, which have a flat number count slope. However, the lensing signal is significantly higher than expected in conventional models. The second possibility is that inter-galactic dust absorption is responsible. A reddening of only $E(B-V)\simeq0.02$ is required to produce the observed correlation. The large 2dF and SDSS QSO surveys should allow a definitive solution to the question of QSO-galaxy correlations.

Keywords: gravitational lensing -- galaxies: statistics -- quasars: general -- dust, extinction