Unveiling the population of dual and lensed active galactic nuclei at sub-arcsec separations

All cosmological models of structure formation predict the existence of a widespread population of dual supermassive black holes in-spiralling inside their common host galaxy, eventually merging and giving rise to intense gravitational waves. These systems can be identified as dual active galactic nuclei (AGNs) at kiloparsec separations, but only very few have been confirmed at z > 0.5. The appearance of multiple AGNs at small angular separations can also be due to gravitational lensing of single AGNs, which are themselves very important systems for many astrophysical topics. Here we present a novel technique, dubbed the Gaia multipeak method, to obtain large and reliable samples of dual/lensed AGN candidates with sub-arcsec separations by looking for AGNs showing multiple peaks in the light profiles observed by the Gaia satellite. All of the Gaia multipeak method-selected sources with high-resolution images (26 from the Hubble Space Telescope archive and 5 from dedicated adaptive-optics-assisted imaging at the Large Binocular Telescope) show multiple components with sub-arcsec separation pointing toward a very high reliability of the method. By sampling separations down to ~2 kpc at z > 1, this method allows us to probe the physical processes that drive the in-spiralling of a pair of supermassive black holes inside a single galaxy.