The major merger–active galactic nucleus connection up to cosmic noon

¹ SRON Netherlands Institute for Space Research, Landleven 12, 9747 AD, Groningen, The Netherlands
² Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV, Groningen, The Netherlands
³ European Space Agency/ESTEC, Keplerlaan 1, 2201 AZ, Noordwijk, The Netherlands
⁴ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, A.P. 72-3, 58089, Morelia, Mexico
⁵ School of Physics and Astronomy, University of Nottingham, University Park, Nottingham, NG7 2RD, UK

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 16 April 2025
Accepted: 26 February 2026

Abstract

Galaxy major mergers are a potential mechanism for triggering active galactic nuclei (AGN) activity, but their role remains debated, particularly beyond the local Universe. We aim to shed light on the merger–AGN connection at z = 0.5–2, exploiting the multi-wavelength datasets and James Webb Space Telescope (JWST) observations in the COSMOS field. We construct a stellar mass-limited sample and identify AGN via mid-infrared (MIR) colours, X-ray detections, and spectral energy distribution (SED) fitting. We train convolutional neural networks to identify mergers with mock JWST observations. We create non-AGN and non-merger control samples matching the redshift, stellar mass, and star formation rate distributions of the AGN and mergers. We find AGN to be somewhat more frequent in mergers than in non-mergers, with excess ratios ranging from ∼2.5 (X-ray AGN) to ∼1.3 (MIR) and ∼1.1–1.2 (SED AGN). Similarly, AGN galaxies show a higher merger fraction (f_merg) than non-AGN controls. We then study f_merg as a function of relative and absolute AGN power, utilising the AGN fraction (f_AGN) and accretion disc luminosity (L_disc) parameters. We uncover a f_merg–f_AGN relation with two regimes: f_merg stays roughly flat for less-dominant AGN (f_AGN < 0.8) but increases at f_AGN > 0.8 for the MIR and X-ray AGN, and more gently for SED AGN, where mergers appear to be the main triggering mechanism. Additionally, f_merg increases monotonically as a function of L_disc, for all AGN types, reaching f_merg > 50% for the most luminous AGN (L_disc ≳ 10⁴⁶ erg s⁻¹). Overall, our results suggest that major mergers can trigger AGN out to cosmic noon at z ∼ 2. Furthermore, the role of major mergers shows a clear dependence on AGN luminosity and remains the principal mechanism for fuelling the most powerful AGN.