Unveiling the population of dual- and lensed- AGNs at sub-arcsec
  separations with Gaia
  [Abridged] We present a novel method that takes advantage of the unique
capabilities of the Gaia satellite to obtain large and reliable samples of dual
or lensed AGN candidates with sub-arcsec separations, by selecting targets
showing multiple peaks in the Gaia scans. With this method, we have identified
260 multiple system candidates, 26 of which have archival Hubble Space
Telescope (HST) images. All these images show two or more point-sources at
sub-arcsec separations, thus confirming that the proposed technique can be
extremely efficient at small separations. Thirteen out of these 26 systems are
known gravitational lenses, and two have been previously classified as dual
AGNs. One of these two systems has an HST/STIS spatially resolved spectrum
showing two distinct AGNs at z=2.95 with 3.6 kpc (0.46 arcsec) separation,
confirming the previous identification as dual AGN. Dedicated adaptive-optics
(AO) assisted high-resolution imaging at the Large Binocular Telescope (LBT) of
5 systems also detected multiple components in all the targets, further
confirming the selection method. The measured separations are between 0.33 and
0.66 arcsec (2.7 - 5.6 kpc at the distance of the primary AGN). The nature of
the unclassified sources is uncertain. Several tests demonstrate that
contamination from foreground stars can only account for ~30% of these multiple
systems, and the analysis of the observed colours suggests that most of the
selected systems are expected to be dual/lensed AGNs with physical separations
between 3 and 5 kpc. Our results show that this is a very efficient technique
to select compact dual/lensed AGN systems with sub-arcsec separations,
complementing other methods. This method samples separation down to 2 kpc at
z>1 and thus allows us to probe the physical processes driving the inspiralling
of the pairs of SMBH inside a single galaxy.
