LoTSS of dual AGN: enhanced detectability and prospects for a systematic search

Q. D'Amato, F. Mannucci, E. De Rubeis, I. Prandoni, A. De Rosa, M. Scialpi, M. Kunert-Bajraszewska, A. Janiuk, A. Krauze, L. Battistini, E. Bertola, C. Bracci, A. Chakraborty, M. Ceci, C. Cicone, G. Cresci, A. Marconi, C. Marconcini, E. Nardini, M. Parvatikar, A. Peca, K. Rubinur, P. Severgnini, C. Spingola, L. Ulivi, G. Venturi, C. Vignali, M. V. Zanchettin

First listed 2026-09-22 | Last updated 2026-09-18

Abstract

Context: Dual active galactic nuclei (DAGN) - systems of two accreting supermassive black holes at kpc-scale separations - trace galaxy mergers and are the progenitors of gravitational-wave sources targeted by pulsar timing arrays and the upcoming Laser Interferometer Space Antenna. However, their radio properties remain poorly constrained, particularly at high redshift. Aims: We demonstrate the detectability of high-redshift DAGN at 144 MHz through sub-arcsec imaging of the LOFAR Two-metre Sky Survey (LoTSS) and characterize their low-frequency radio properties. Methods: We cross-matched 92 spectroscopically confirmed DAGN (z > 0.3, projected separation r_p < 30 kpc) with the LoTSS DR2 catalog and reprocessed international LOFAR telescope (ILT) data for three systems (0.59 < z < 2.39), achieving ~0.3 arcsec resolution at 144 MHz. Multi-frequency analysis and brightness-temperature diagnostics were used to investigate the radio emission. Results: We obtained high-quality ILT images for all three systems. In one source at z = 1.49, both AGN are detected at >5 sigma, making it one of the highest-redshift and faintest radio-detected DAGN. Brightness temperatures confirm AGN-powered emission even in sub-mJy sources. In the other two systems only one AGN is detected; for one, the new data constrain the low-frequency spectrum of a compact steep-spectrum source, indicating that its morphology is driven by jet-ISM interaction rather than the companion galaxy. For the 92 objects sample, the LoTSS detection rate is ~24%, significantly higher than for matched single AGN (~9-10%), suggesting an enhanced detectability of DAGN in the LoTSS. Conclusions: Low-frequency sub-arcsec radio observations provide an efficient, dust-unbiased method to detect and characterize DAGN. This pilot study establishes a benchmark for the forthcoming all-sky ~0.3 arcsec iLoTSS survey and future SKA observations.

Short digest

D’Amato et al. test whether sub-arcsecond 144 MHz LOFAR imaging can identify and characterize kpc-scale dual AGN, cross-matching 92 spectroscopically confirmed systems at z > 0.3 and reprocessing international-LOFAR data for three targets at 0.59 < z < 2.39. The pilot resolves both nuclei in J1120+6711 at z = 1.49 at >5σ, making it among the highest-redshift and faintest radio-detected dual AGN, while brightness temperatures show that even sub-mJy low-frequency emission can be AGN-powered. Only one nucleus is detected in each of J0930+4614 and J1643+3156; in the latter, the compact steep-spectrum morphology is attributed to jet–ISM interaction rather than the companion. Across the parent sample, LoTSS detects about 24% of dual AGN versus 9–10% of matched single AGN, positioning iLoTSS-like imaging as a dust-unbiased route toward systematic dual-AGN searches and SKA-era samples.

Key figures to inspect

• Figure 1. This is the core observational result: 0.3-arcsec 144 MHz ILT maps show both radio AGN in J1120+6711 while marking the undetected companions in J0930+4614 and J1643+3156. It directly conveys both the demonstrable resolving power for a z = 1.49 dual detection and the incompleteness implied by single-component detections.
• Figure 3. The component-by-component radio spectra of J1643+3156 provide the physical diagnostic behind the paper’s interpretation of this system. They show how the new 144 MHz constraints distinguish core and hotspot behavior and support jet–ISM interaction, rather than the companion galaxy, as the origin of its complex radio morphology.
• Figure 4. This synthesis figure connects the pilot to survey-scale prospects by comparing simulated dual-AGN surface densities with expected 144 MHz and 1.4 GHz flux densities and LoTSS/SKA detection limits. It makes clear why sensitive, wide-area low-frequency imaging can turn individual dual-AGN detections into a statistically useful census.