A Large Systematic Search for Recoiling and Close Supermassive Binary
  Black Holes
  [ABRIDGED] We have carried out a systematic search for close supermassive
black hole binaries among z < 0.7 SDSS quasars Such binaries are predicted by
models of supermassive black hole and host galaxy co-evolution, therefore their
census and population properties constitute an important test of these models.
We used an automatic technique based on spectroscopic principal component
analysis to search for broad H-beta lines that are displaced from the
rest-frame of the quasar by more than 1,000 km/s This method can also yield
candidates for rapidly recoiling black holes. Our search yielded 88 candidates,
several of which were previously identified and discussed in the literature.
The widths of the broad H-beta lines are typical among quasars but the shifts
are extreme. We found a correlation between the peak offset and skewness of the
broad H-beta profiles, which suggests that the profiles we have selected share
a common physical explanation. The general properties of the narrow emission
lines are typical of quasars. We carried out followup spectroscopic
observations of 68 objects to search for changes in the peak velocities of the
H-beta lines (the time interval in the observer's frame between the original
and new observations is 1-10 yr). We measured significant changes in 14
objects, with resulting accelerations between -120 and +120 km/s/yr. We
emphasize that interpretation of the offset broad emission lines as signatures
of supermassive binaries is subject to many significant caveats. Many more
followup observations over a long temporal baseline are needed to characterize
the variability pattern of the broad lines and test that this pattern is indeed
consistent with orbital motion. The possibility that some of the objects in
this sample are rapidly recoiling black holes remains open as the available
data do not provide strong constraints for this scenario.
