Galaxy interactions are the dominant trigger for local type 2 quasars. Issue 2 (13th February 2023)

• Record Type: Journal Article
• Authors: Pierce, J C S; Tadhunter, C; Ramos Almeida, C; Bessiere, P; Heaton, J V; Ellison, S L; Speranza, G; Gordon, Y; O'Dea, C; Grimmett, L; Makrygianni, L
• Is Part Of: Monthly notices of the Royal Astronomical Society. Volume 522:Issue 2(2023)
• Page Start: 1736
• Page End: 1751
• Publication Date: 2023-02-13
• Subjects: galaxies: active -- galaxies: interactions -- galaxies: nuclei; Astronomy -- Periodicals; Periodicals; 520.5
• DOI: 10.1093/mnras/stad455
• Languages: English
• ISSNs: 0035-8711

Abstract

ABSTRACT: 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 ($L_{\rm [O\, \small {III}]}\gt 10^{8.5}\, \mathrm{L}_{\odot }$ ; 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 5σ 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 [O iii ] 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.