Interacting galaxies in the IllustrisTNG simulations - VII: the connection between the most luminous active galactic nuclei and galaxy interactions

We investigate the connection between the most luminous active galactic nuclei (AGN), galaxy pairs, and post-mergers in the IllustrisTNG simulation. We select galaxy pairs and post-mergers with a mass ratio between 1:10 < mu < 1:1 and a redshift between 0 < z < 1. We compare the incidence of luminous AGN in pairs with matched non-pair controls, finding that AGN with luminosity L-bol > 10(44) erg s(-1) occur in similar to 26 per cent of paired galaxies with a companion within 20 kpc, compared with similar to 14 per cent in controls (a fractional excess of similar to 2). The enhancement of AGN in galaxy pairs is luminosity dependent, with the highest excess (a factor of similar to 6 +/- 2 at the closest separations) for AGN with L-bol > 10(45) erg s(-1). Additionally, pairs exhibit a modest yet statistically significant excess of luminous AGN up to similar to 150 kpc in separation. For pairs which merge between 0 < z < 1, AGN fractions are elevated between 1.5 Gyr before and after coalescence, with the highest excess closest to coalescence. Our results indicate that pre-coalescence interactions drive excesses of luminous AGN, but that luminous AGN in galaxy pairs are not ubiquitous. Finally, we investigate what fraction of AGN can be associated with an interaction (either having a companion within 100 kpc or a merger within the last 500 Myr). For AGN with L-bol > 10(45) erg s(-1), similar to 55 per cent are interacting, compared with a 30 per cent interaction fraction in AGN with 10(44) < L-bol < 10(44.5) erg s(-1). Our results support a picture in which interactions play a dominant role in (but are not the sole cause of) triggering the most luminous AGN.