Dual supermassive black holes at close separation revealed by the Hyper
  Suprime-Cam Subaru Strategic Program
  The unique combination of superb spatial resolution, wide-area coverage, and
deep depth of the optical imaging from the Hyper Suprime-Cam (HSC) Subaru
Strategic Program is utilized to search for dual quasar candidates. Using an
automated image analysis routine on 34,476 known SDSS quasars, we identify
those with two (or more) distinct optical point sources in HSC images covering
796 deg^2. We find 421 candidates out to a redshift of 4.5 of which one hundred
or so are more likely after filtering out contaminating stars. Angular
separations of 0.6 - 4.0" correspond to projected separations of 3 - 30 kpc, a
range relatively unexplored for population studies of luminous dual quasars.
Using Keck-I/LRIS and Gemini-N/NIFS, we spectroscopically confirm three dual
quasar systems at z < 1, two of which are previously unknown out of eight
observed, based on the presence of characteristic broad emission lines in each
component, while highlighting that the continuum of one object in one of the
pairs is reddened. In all cases, the [OIII]5007 emission lines have mild
velocity offsets, thus the joint [OIII] line profile is not double-peaked. We
find a dual quasar fraction of 0.26+/-0.18% and no evidence for evolution. A
comparison with the Horizon-AGN simulation seems to support the case of no
evolution in the dual quasar fraction when broadly matching the quasar
selection. These results may indicate a scenario in which the frequency of the
simultaneous triggering of luminous quasars is not as sensitive as expected to
the cosmic evolution of the merger rate or gas content of galaxies.
