Galaxy interactions are the dominant trigger for local type 2 quasars

The triggering mechanism for the most luminous, quasar-like active galactic nuclei (AGN) remains a source of debate, with some studies favouring triggering via galaxy mergers, but others finding little evidence to support this mechanism. Here, we present deep Isaac Newton Telescope/Wide Field Camera imaging observations of a complete sample of 48 optically selected type 2 quasars - the QSOFEED sample ( L-[OIII] > 10 (8 . 5 )L(?)z < 0.14). Based on visual inspection by eight classifiers, we find clear evidence that galaxy interactions are the dominant triggering mechanism for quasar activity in the local universe, with 65(-7)( + 6 ) per cent of the type 2 quasar hosts showing morphological features consistent with galaxy mergers or encounters, compared with only 22 (+ 5) (-4) per cent of a stellar -mass-and redshift-matched comparison sample of non-AGN galaxies - a 5 s difference. The type 2 quasar hosts are a factor of 3.0 (+ 0 . 5)( -0 .8) more likely to be morphologically disturbed than their matched non-AGN counterparts, similar to our previous results for powerful 3CR radio AGN of comparable [OIII] emission-line luminosity and redshift. In contrast to the idea that quasars are triggered at the peaks of galaxy mergers as the two nuclei coalesce, and only become visible post-coalescence, the majority of morphologically disturbed type 2 quasar sources in our sample are observed in the pre-coalescence phase (61 + 8 -9 per cent). We argue that much of the apparent ambiguity that surrounds observational results in this field is a result of differences in the surface brightness depths of the observations, combined with the effects of cosmological surface brightness dimming.