Binary Quasars at High Redshift I: 24 New Quasar Pairs at z ~ 3-4
  The clustering of quasars on small scales yields fundamental constraints on
models of quasar evolution and the buildup of supermassive black holes. This
paper describes the first systematic survey to discover high redshift binary
quasars. Using color-selection and photometric redshift techniques, we searched
8142 deg^2 of SDSS imaging data for binary quasar candidates, and confirmed
them with follow-up spectroscopy. Our sample of 27 high redshift binaries (24
of them new discoveries) at redshifts 2.9 < z < 4.3 with proper transverse
separations 10 kpc < R_{\perp} < 650 kpc increases the number of such objects
known by an order of magnitude. Eight members of this sample are very close
pairs with R_{\perp} < 100 kpc, and of these close systems four are at z > 3.5.
The completeness and efficiency of our well-defined selection algorithm are
quantified using simulated photometry and we find that our sample is ~ 50%
complete. Our companion paper uses this knowledge to make the first measurement
of the small scale clustering (R < 1 Mpc/h comoving) of high-redshift quasars.
High redshift binaries constitute exponentially rare coincidences of two
extreme (M >~ 10^9 Msun) supermassive black holes. At z ~ 4 there is about one
close binary per 10 Gpc^3, thus these could be the highest sigma peaks, the
analogs of superclusters, in the early Universe.
