A hidden population of high-redshift double quasars unveiled by
  astrometry
  Galaxy mergers occur frequently in the early universe and bring multiple
supermassive black holes (SMBHs) into the nucleus, where they may eventually
coalesce. Identifying post-merger-scale (i.e., <~a few kpc) dual SMBHs is a
critical pathway to understanding their dynamical evolution and successive
mergers. While serendipitously discovering kpc-scale dual SMBHs at z<1 is
possible, such systems are elusive at z>2, but critical to constraining the
progenitors of SMBH mergers. The redshift z~2 also marks the epoch of peak
activity of luminous quasars, hence probing this spatial regime at high
redshift is of particular significance in understanding the evolution of
quasars. However, given stringent resolution requirements, there is currently
no confirmed <10 kpc physical SMBH pair at z>2. Here we report two sub-arcsec
double quasars at z>2 discovered from a targeted search with a novel
astrometric technique, demonstrating a high success rate (~50%) in this
systematic approach. These high-redshift double quasars could be the
long-sought kpc-scale dual SMBHs, or sub-arcsec gravitationally-lensed quasar
images. One of these double quasars (at z=2.95) was spatially resolved with
optical spectroscopy, and slightly favors the scenario of a physical quasar
pair with a projected separation of 3.5 kpc (0.46"). Follow-up observations of
double quasars discovered by this targeted approach will be able to provide the
first observational constraints on kpc-scale dual SMBHs at z>2.
