Galaxy interactions are the dominant trigger for local type 2 quasars

• Published in: arXiv.org 2023-03
• Main Authors: Pierce, J C S, Tadhunter, C, C Ramos Almeida, Bessiere, P, Heaton, J V, Ellison, S L, Speranza, G, Gordon, Y, O'Dea, C, Grimmett, L, Makrygianni, L
• Format: Article
• Language: English
• Subjects: Active galactic nuclei; Dimming; Field cameras; Galaxy mergers & collisions; Inspection; Luminosity; Morphology; Quasars; Red shift; Surface brightness
• ISSN: 2331-8422
• DOI: 10.48550/arxiv.2303.15506

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\(_{\rm [OIII]}>\)10$^{8.5}$$L_{\odot}\(; \)z < 0.14\(). Based on visual inspection by eight classifiers, we find clear evidence that galaxy interactions are the dominant triggering mechanism for quasar activity in the local universe, with 65\)^{+6}_{-7}\( per cent of the type 2 quasar hosts showing morphological features consistent with galaxy mergers or encounters, compared with only 22\)^{+5}_{-4}\( per cent of a stellar-mass- and redshift-matched comparison sample of non-AGN galaxies \)-\( a 5\)\sigma\( difference. The type 2 quasar hosts are a factor 3.0\)^{+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.