Powerful Radio-loud Quasars Are Triggered by Galaxy Mergers in the Cosmic Bright Ages

Abstract

While supermassive black holes are ubiquitous features of galactic nuclei, only a small minority are observed during episodes of luminous accretion. The physical mechanism(s) driving the onset of fueling and ignition in these active galactic nuclei (AGN) are still largely unknown for many galaxies and AGN-selection criteria. Attention has focused on AGN triggering by means of major galaxy mergers gravitationally funneling gas toward the galactic center, with evidence both for and against this scenario. However, several recent studies have found that radio-loud AGN overwhelmingly reside in ongoing or recent major galaxy mergers. In this study, we test the hypothesis that major galaxy mergers are important triggers for radio-loud AGN activity in powerful quasars during cosmic noon (1 ≲ z ≲ 2). To this end, we compare Hubble Space Telescope WFC3/IR observations of the z > 1 3CR radio-loud broad-lined quasars to three matched radio-quiet quasar control samples. We find strong evidence for major-merger activity in nearly all radio-loud AGN, in contrast to the much lower merger fraction in the radio-quiet AGN. These results suggest major galaxy mergers are key ingredients in launching powerful radio jets. Given many of our radio-loud quasars are blue, our results present a possible challenge to the “blowout” paradigm of galaxy evolution models in which blue quasars are the quiescent end result following a period of red quasar feedback initiated by a galaxy merger. Finally, we find a tight correlation between black hole mass and host galaxy luminosity for these different high-redshift AGN samples that is inconsistent with those observed for local elliptical galaxies.

Prevalence of Compact Nuclear Radio Emission in Post-merger Galaxies and Its Origin

“…We emphasize important distinctions between our study and those focusing on the incidence of galaxy mergers in radioemitting systems. These studies (e.g., Chiaberge et al 2015;Breiding et al 2024) placed constraints on the merger fraction of AGN host galaxies; they began by selecting for a sample of luminous quasars and examined the host-galaxy morphology for signs of ongoing or recent gravitational interaction. Our sample, however, is of known merging galaxies that were selected because of their host-galaxy morphology and the presence of a single optical nucleus (C12).…”

“…Observational studies are in conflict with one another over the role mergers play in triggering an AGN. While many have found either an increased AGN fraction in merging systems (Ellison et al 2011;Satyapal et al 2014;Donley et al 2018;Goulding et al 2018) or an increased merger fraction in AGN hosts (Chiaberge et al 2015;Fan et al 2016;Gao et al 2020;Marian et al 2020;Breiding et al 2024), others have found no such connection between AGNs and mergers (Grogin et al 2005;Cisternas et al 2011;Böhm et al 2013;Villforth et al 2017;Marian et al 2019;Lambrides et al 2021). Selection biases almost certainly contribute to this dissonance.…”

“…Selection biases almost certainly contribute to this dissonance. Different AGN selection criteria, e.g., mid-infrared (mid-IR; e.g., Satyapal et al 2014;Donley et al 2018;Goulding et al 2018), X-ray (e.g., Grogin et al 2005;Villforth et al 2017), optical (e.g., Böhm et al 2013), and radio (e.g., Chiaberge et al 2015;Breiding et al 2024), and selection of mergers at various stages of their evolution also necessarily select different astrophysical scenarios (e.g., Sanders et al 1988).…”