The author Xiaohui Fan in a quasar presentation mentioned a recent discovery of quasar clustering and quasar binaries (This I believe is the paper he was referring to.) Fan's presentation was the first I have heard of quasar clustering. I did not know there were quasar binaries. I am not sure what to make of this finding.
http://arxiv.org/abs/0709.3474
"Quasar Clustering from a Complete Sample of Binaries" by Adam Myers, Gordon Richards, Robert Brunner, Donald Schneider, Natalie Strand, Patrick Hall, Jeffrey Blomquist, and Donald York
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Our data thus imply excess quasar clustering at ∼ 25 h−1 kpc of a factor of ∼ 4, consistent with the quoted upper limit in M07b. Given that our sample is targeted differently than any previous samples of binary quasars (i.e. UVX but otherwise regardless of the color similarity of the candidate components), and given our simple selection function, our work might be viewed as independently corroborating the evidence for excess quasar clustering on small scales first detected in H06.
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Of the 22 new pairs, 10 turn out to be binary quasars (of which ∼ 2 might actually be lenses). This roughly doubles the number of known binary quasars with (my comment binary separation) is 3′′ ≤ theta < 6′′ at z ∼ < 2 and g < 21. A further 9 of our observed candidates are projected quasar pairs, and 3 contain a NELG or star. This confirms that the KDE technique is ∼ 95% efficient at selecting quasars (e.g., Richards et al. 2004; Myers et al. 2006, 2007a). Combined with observations from the literature (mainly from H06), 46% of the DR4 KDE binary quasar candidates have now been observed, of which ∼ 47% are binaries or lenses, ∼ 40% are projected quasar pairs, and the remainder contain a nonquasar.
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