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QSO-Galaxy Correlations: Lensing or Dust?
Croom, Scott M.
AA(Anglo-Australian Observatory, PO Box 296, Epping, NSW 2121, Australia)
Publications of the Astronomical Society of Australia, Volume 18, Issue 2, pp. 169-171. (PASA Homepage)
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Astronomy Keywords:
gravitational lensing, galaxies: statistics, quasars: general, dust, extinction
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We present evidence for an anti-correlation between faint QSOs and B < 23 galaxies. A sample of 192 QSOs in a 2.5 deg2 area has been imaged using the Isaac 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) ~= 0.02 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.

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