The complex relationships between active galactic nuclei, bars, and bulges

¹ Department of Theoretical Physics and Astrophysics, Faculty of Science, Masaryk University, Kotlářská 2 Brno 611 37, Czech Republic
² Department of Physics, Lancaster University, Lancaster LA1 4YB, UK
³ School of Physics and Astronomy, University of Minnesota, Minneapolis, Minnesota 55455, USA
⁴ Minnesota Institute for Astrophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA
⁵ Dunlap Institute for Astronomy and Astrophysics, University of Toronto, 50 St. George Street, Toronto, ON M5S 3H4, Canada
⁶ Oxford Astrophysics, Department of Physics, University of Oxford, Denys Wilkinson Building, Keble Road, Oxford OX1 3RH, UK
⁷ European Space Agency (ESA), European Space Astronomy Centre (ESAC), Camino Bajo del Castillo s/n, 28692 Villaneuva de la Cañada, Madrid, Spain
⁸ Jodrell Bank Centre for Astrophysics, Department of Physics & Astronomy, University of Manchester, Oxford Road, Manchester M13 9PL, UK

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

Received: 20 October 2025
Accepted: 28 March 2026

Abstract

Context. Via scaling relations, it is well known that active galactic nuclei (AGNs) and bulges are linked. This link was thought to be driven by mergers, but recent studies show that secular processes are the dominant mechanism of supermassive black hole growth. One such secular mechanism is gas inflow driven by large-scale bars. Since bulges can also grow via these bars, these three features likely share some common process.

Aims. We investigated whether the observed correlation between AGNs and bars is real or arises as a result of correlations between bars and bulges.

Methods. Using a catalogue of AGN identifications and galaxy morphologies in the DESI Legacy Survey at z ≤ 0.1, we controlled for mass and colour and investigated the AGN fraction variation with bulge prominence and bar strength.

Results. We first show that the variation in the AGN fraction between strongly barred, weakly barred, and unbarred galaxies does not qualitatively change if we additionally control for bulge prominence. Second, we find that in fixed bins of bulge prominence, the AGN fraction increases with increasing bar strength. In subsamples split by bar strength, the AGN fraction increases with bulge prominence, indicating that AGN presence correlates with bar strength and bulge prominence simultaneously.