Dust and power: Unravelling the merger-active galactic nucleus connection in the second half of cosmic history

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 Department of Astronomy, Nanjing University, Nanjing, 210093 China
⁴ Key Laboratory of Modern Astronomy and Astrophysics (Nanjing University), Ministry of Education, Nanjing, 210093 China
⁵ Department of Astrophysical Sciences, Princeton University, 4 Ivy Lane, Princeton, NJ, 08544 USA
⁶ Korea Astronomy and Space Science Institute, 776 Daedeok-daero, Yuseong-gu, Daejeon, 34055 Korea
⁷ Steward Observatory, University of Arizona, 933 N. Cherry Ave, Tucson, AZ, 85719 USA
⁸ School of Physics and Astronomy, University of Nottingham, University Park, Nottingham, NG7 2RD UK
⁹ Instituto de Radioastronomía y Astrofísica, Universidad Nacional Autónoma de México, A.P. 72-3, 58089 Morelia, Mexico
¹⁰ School of Physics and Astronomy, University of Edinburgh, Edinburgh, EH9 3HJ UK
¹¹ Department of Physics and Astronomy, University of the Western Cape, Bellville, 7535 South Africa
¹² Institut d’Astrophysique de Paris, Sorbonne Université, CNRS, UMR 7095, 98 bis bd Arago, 75014 Paris, France

Received: 6 October 2023
Accepted: 14 August 2024

Abstract

Aims. Galaxy mergers represent a fundamental physical process under hierarchical structure formation, but their role in triggering active galactic nuclei (AGNs) is still unclear. We aim to investigate the merger-AGN connection using state-of-the-art observations and novel methods for detecting mergers and AGNs.

Methods. We selected stellar mass-limited samples at redshift z < 1 from the Kilo-Degree Survey (KiDS), focussing on the KiDS-N-W2 field with a wide range of multi-wavelength data. We analysed three AGN types, selected in the mid-infrared (MIR), X-ray, and via spectral energy distribution (SED) modelling. To identify mergers, we used convolutional neural networks (CNNs) trained on two cosmological simulations. We created mass- and redshift-matched control samples of non-mergers and non-AGNs.

Results. We first investigated the merger-AGN connection using a binary AGN/non-AGN classification. We observed a clear AGN excess (of a factor of 2–3) in mergers with respect to non-mergers for the MIR AGNs, along with a mild excess for the X-ray and SED AGNs. This result indicates that mergers could trigger all three types, but are more connected to the MIR AGNs. About half of the MIR AGNs are in mergers but it is unclear whether mergers are the main trigger. For the X-ray and SED AGNs, mergers are unlikely to be the dominant triggering mechanism. We also explored the connection using the continuous AGN fraction f_AGN parameter. Mergers exhibit a clear excess of high f_AGN values relative to non-mergers, for all AGN types. We unveil the first merger fraction f_merger − f_AGN relation with two distinct regimes. When the AGN is not very dominant, the relation is only mildly increasing or even flat, with the MIR AGNs showing the highest f_merger. In the regime of very dominant AGNs (f_AGN ≥ 0.8), f_merger shows the same steeply rising trend with increasing f_AGN for all AGN types. These trends are also seen when plotted against AGN bolometric luminosity. We conclude that mergers are most closely connected to dust-obscured AGNs, generally linked to a fast-growing phase of the supermassive black hole. Such mergers therefore stand as the main (or even the sole) fuelling mechanism of the most powerful AGNs.