Optical-to-near-infrared studies of AGN and dual AGN systems with Euclid and the HSC-Niji survey

Supermassive black holes (SMBHs), almost ubiquitous in galaxies, are polyglot citizens of the Universe: we observe them igniting as Active Galactic Nuclei (AGN) across the electromagnetic spectrum, and potentially hear the ripples in spacetime, the gravitational waves (GW), they create in collisions. The upcoming Laser Interferometer Space Antenna (LISA) mission will be sensitive to the GW signals from SMBH coalescence, complementing Pulsar Timing Array (PTA) observatories. The ESA Euclid space mission, surveying billions of galaxies in the optical to near-infrared (NIR) with unmatched high-resolution imaging and wide field of view, will bring remarkable advances in the study of the co-evolutionary path of SMBHs and their host galaxies. A fundamental yet poorly explored stage of this evolution is represented by dual AGN systems, which trace the pathway to SMBH coalescence, and can be used to set empirical priors for LISA GW events. Despite their high scientific value, instrumental challenges have limited the number of dual AGN detections confirmed so far, as exquisite spatial and spectral resolution is needed to characterise the galaxy nuclear light, and most surveys lack the necessary statistical power. In this talk, I will discuss recent efforts to build up the first systematic census of dual AGN in Euclid. I will present a method for identifying dual AGN candidates using photometric data from the Euclid Q1 release. Our approach is based on the computation of the overlap integral between the redshift probability distribution functions (PDZs) of AGN and their close companions. This probabilistic criterion naturally incorporates photometric uncertainties and enables a robust selection of physically associated systems, overcoming the limitations of simple redshift difference cuts. We analyze the redshift and projected distance distributions of these systems, which are key elements for comparison with theoretical expectations for dual AGN formation and the implications for future gravitational wave detections with LISA. We also study their physical properties, focusing on their occurrence as a function of the stellar mass of the host. I will also discuss the possibility to advance our understanding of AGN activity and feedback by exploiting unprecedented synergies with HSC-Niji, a new medium-band survey with the Subaru telescope. The latter will also enable the study of AGN-linked populations, such as Little Red Dots, as well as cosmology-focused analyses, including the multi-wavelength characterisation of galaxy shapes and more accurate galaxy SED reconstruction and photo-z estimates, all relevant for weak-lensing analyses in current and future Stage IV surveys.