Galactic bars and active galactic nucleus fuelling 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 European Space Agency (ESA), European Space Astronomy Centre (ESAC) Camino Bajo del Castillo s/n 28692 Villaneuva de la Cañada Madrid, Spain

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Received: 3 July 2025
Accepted: 8 January 2026

Abstract

We investigate the role of galactic bars in fuelling and triggering active galactic nuclei (AGN) in disc galaxies up to z ∼ 0.8. We utilised a deep learning model, fine-tuned on Galaxy Zoo volunteer classifications, to identify (strongly and weakly) barred and unbarred disc galaxies in Hyper Suprime-Cam Subaru Strategic Program i-band images. We selected AGN using three independent diagnostics: mid-infrared colours, X-ray detections, and spectral energy distribution (SED) fitting. The SED analysis, performed using CIGALE, quantifies the relative AGN contribution to the total galaxy luminosity (f_AGN) and the AGN luminosity (L_disc). We assessed the impact of bars by comparing AGN incidence and properties in barred galaxies against carefully constructed redshift-, stellar mass-, and colour-matched unbarred control samples. Our binary AGN classification experiment demonstrates that barred disc galaxies host a higher fraction of AGN compared to their unbarred counterparts, though the significance depends on the AGN selection method, with a more modest excess for SED AGN, and control sample size. This suggests a contributing role for bars in the global AGN budget. The contribution of bars to AGN fuelling appears confined to systems where the AGN has a lower relative contribution to the host galaxy’s emission (f_AGN < 0.75). Crucially, we find a significant dearth of barred disc galaxies hosting AGN with f_AGN > 0.75, independent of bar strength. Consistent with this, the fraction of barred galaxies among AGN hosts decreases with increasing L_disc. Combined with previous results, we suggest that bars may contribute to fuelling the population of low-to-moderate-luminosity AGN, but major mergers are the principal mechanism for triggering the most powerful and dominant accretion events.