2008ApJ...678..635M

We present spectroscopy of binary quasar candidates, with component separations of 3"≤Δθ<6", selected from Data Release 4 of the Sloan Digital Sky Survey (SDSS DR4) using kernel density estimation (KDE). Of our 27 new quasar pair observations, 10 are binary quasars, which doubles the number of known g<21 binaries with 3"≤Δθ<6" separations. Several of our observed binaries are wide-separation lens candidates that merit additional higher resolution spectroscopy, as well as deep imaging to search for lensing galaxies. Our candidates are initially selected by UV excess (u-g<1), but are otherwise selected irrespective of the relative colors of the quasar pair, and we thus use them to suggest optimal color similarity and photometric redshift approaches for targeting binary quasars or projected quasar pairs. We find that a third or more of all binary quasars have quite dissimilar components on the basis of a typical color similarity criterion (χ²_color≲20). From a sample that is complete on proper scales of 23.7 h^–1 kpc<R_prop<29.9 h^–1 kpc, we determine the projected quasar correlation function to be W{d1}_p=24.0^+16.9_–10.8, which is 2 σ lower than recent estimates. We argue that our low W{d1}_pestimates may indicate redshift evolution in the quasar correlation function from z∼1.9 to z∼1.4 on scales of R_prop∼25 h^–1 kpc. The size of this evolution broadly tracks quasar clustering on larger scales, consistent with merger-driven models of quasar origin. Spectroscopy of all of our DR4 KDE binary quasar candidates should be sufficient to detect quasar clustering evolution at R_prop∼25 h^–1 kpc for z<2.5 in a single homogeneous sample.