Dual and lensed AGN at sub-arcsecond separations uncovered by ERIS

We present the first results of the ongoing INAF GTO programme focused on the search of sub--arcsecond dual active galactic nuclei (AGN), carried out with the near-infrared integral-field unit SPIFFIER of the Enhanced Resolution Imager and Spectrograph (ERIS) at the ESO Very Large Telescope, operating in adaptive-optics mode with a laser guide star. We report integral field spectroscopy with the low--resolution gratings (R ∼ 5000 - 5600) of 34 multiple AGN candidates selected from the Gaia Data Release 3 source catalogue via the Gaia Multipeak (GMP) method, which preferentially identifies luminous unobscured AGN at sub-arcsecond separations. The targets span the redshift range of 0.7 łesssim z łesssim 3.4 and exhibit projected angular separations between ∼0.2'' and ∼0.9'', corresponding to physical separations of ∼1.7--7.8 kpc. Of the 34 observed systems,13 (∼40%) are resolved into multiple AGN-like components; and 18 (∼53%) are classified as AGN plus foreground star superpositions, with one as a single AGN, and two as systems with an AGN primary and an unclassified secondary component. Among the multiple AGN systems, we confirm six dual AGN at projected separations of 0.26'' -- 0.80'' (2.2--6.7 kpc), in the redshift range of z = 0.68 -- 1.57, and with velocity offsets in the range of Δ v ∼ 290 -- 950 ̨ms. We also identify seven gravitationally lensed quasars, only one of which was previously known, including four doubly imaged and three quadruply imaged systems. These results substantially enlarge the census of spectroscopically confirmed compact multiple AGN systems at z > 0.5 and demonstrate the efficiency of ERIS--SPIFFIER for the systematic characterization of sub-arcsecond dual and lensed AGN candidates.