Cosmic Duets I. High-spatial resolution spectroscopy of dual and lensed AGN with MUSE

We present the first-year results of the MUSE Large Program “Cosmic Duets”, aimed at obtaining adaptive-optics assisted MUSE observations with an angular resolution of 0.1"-0.2", providing integral-field spectroscopy of sub-arcsec separation dual and lensed active galactic nuclei (AGN) candidates. These observations reveal previously unexplored properties of dual and lensed systems, key to understanding galaxy evolution, black hole mergers, and strong-lensing modeling. Targets were selected using the Gaia Multi-Peak (GMP) technique, which identifies pairs of point-like sources with separations below 0.8" in the Gaia catalog. MUSE spatially resolved spectroscopy provides redshifts, ionization diagnostics, and absorption systems along the line of sight. We report results for 30 GMP-selected targets at z=0.5-3.5. All systems show at least two spatially resolved components. Nineteen objects are confirmed as AGN multiplets, including 6 dual AGN, 10 doubly-lensed quasars, and 3 quadruply-lensed systems, while the remaining 11 correspond to alignments with foreground stars. Among spectroscopically confirmed dual AGN in the literature, 24 pairs have separations below 7 kpc, and our sample accounts for 25% of them. We study dual and lensed AGN distributions as a function of redshift, magnitude, and projected separation, and find that bright systems (J<16.5) are dominated by lensed quasars, whereas the fraction of dual AGN increases at fainter magnitudes. This first-year sample demonstrates the high efficiency of GMP pre-selection combined with MUSE spectroscopy. The full program, targeting 150 systems, will enable statistical studies of dual AGN incidence and detailed constraints on mass distribution in strong-lensing galaxies.

💡 Research Summary

This paper presents the first-year results from the ESO Large Program “Cosmic Duets,” which aims to systematically discover and characterize sub-arcsecond separation dual active galactic nuclei (AGN) and gravitationally lensed quasars. These objects are crucial for understanding late-stage galaxy mergers, supermassive black hole (SMBH) binary evolution, and the mass distribution in lens galaxies.

The key innovation lies in the target selection method. The team employed the Gaia Multi-Peak (GMP) technique, which identifies double peaks in the light profiles of sources from the Gaia DR3 catalog. This method efficiently selects candidate pairs with angular separations between 0.15" and 0.8", corresponding to physical projected distances of a few kiloparsecs at cosmic noon (z=0.5-3.5). Candidates were drawn from spectroscopic AGN catalogs (e.g., Milliquas) and photometric AGN catalogs, with the latter confirmed via follow-up spectroscopy from telescopes like the NTT and TNG.

The heart of the program is high-spatial-resolution integral-field spectroscopy. Using the Multi Unit Spectroscopic Explorer (MUSE) instrument on the VLT in its adaptive-optics-assisted Narrow-Field Mode, the team achieved an angular resolution of 0.1"-0.2". This allowed them to spatially resolve the two components of each candidate and extract their separate spectra.

The analysis of these spectra led to a definitive classification for each of the 30 first-year targets. The criteria were: 1) Dual AGN: Two distinct AGN spectra with different redshifts and/or independent broad emission lines, indicating two separate SMBHs. 2) Lensed Quasar: Two components with virtually identical AGN spectra, signifying multiple images of a single background quasar. 3) AGN-star alignment: One component shows an AGN spectrum, while the other shows the absorption-line spectrum of a foreground star.

The results are highly productive: out of 30 observed systems, 19 (63%) are confirmed AGN multiples. This includes 6 dual AGN, 10 doubly lensed quasars, and 3 quadruply lensed quasars. The remaining 11 are chance alignments with foreground stars. Notably, the 6 confirmed dual AGN pairs all have projected separations below 7 kpc, representing 25% of all such spectroscopically confirmed close pairs known in the literature.

The study also investigates population trends. They find that bright systems (J < 16.5) are dominated by lensed quasars, a consequence of the magnification boost provided by gravitational lensing. In contrast, the relative fraction of dual AGN increases among fainter systems. This trend may reflect both selection effects and a potentially higher abundance of dual AGN in fainter, merging systems.

The first-year sample robustly demonstrates the high efficiency of combining GMP pre-selection with MUSE follow-up spectroscopy for identifying sub-arcsecond AGN multiples. The success rate of ~63% validates the GMP method as a powerful tool for probing a previously difficult-to-access parameter space. The full “Cosmic Duets” program, which will target approximately 150 systems, promises to enable statistical studies on the incidence and evolution of dual AGN, provide precise constraints on the stellar initial mass function and dark matter content in the central regions of lens galaxies, and ultimately inform models of SMBH binary coalescence and the predicted gravitational wave background for future observatories like LISA.