The dependence of triggering mechanisms on radio AGN sub-types: the role of galaxy mergers

TL;DR

This study analyzes the triggering mechanisms of powerful radio AGN by comparing a near-complete, low-redshift 3CR sample (112 objects, z<0.3) with a stellar-mass matched control sample, using deep optical imaging from INT/WFC and GMOS-S. Morphological classifications from a Zooniverse-based interface reveal robust, statistically significant differences between sub-types: high-excitation HERGs are much more disturbed than low-excitation LERGs, consistent with merger-driven fueling for radiatively efficient AGN, while LERGs align with their controls, suggesting hot-gas accretion as the dominant trigger. FRII HERGs show stronger disturbance than FRII LERGs, implying FRII LERGs are not simply a faded HERG population; quasar-like 3CRs exhibit the strongest merger signatures, reinforcing mergers as a key trigger for the most luminous AGN. The analysis also finds a broad distribution of merger stages, with a majority in post-coalescence phases, and notes observational factors such as depth and extinction that affect detection of tidal features. Overall, the work provides robust evidence for differentiated triggering mechanisms across radio AGN sub-types and highlights the value of complete, mass-matched samples for disentangling host-galaxy evolution and AGN feedback in massive galaxies.

Abstract

Powerful, radio-loud active galactic nuclei (AGN) are associated with one of the most important forms of AGN feedback, and understanding how they are triggered is key to properly incorporating them into models of galaxy evolution. Here, we present the results of a deep Isaac Newton Telescope/Wide Field Camera imaging survey which, when combined with Gemini/Gemini Multi-Object Spectrograph South images, gives a 98 per cent complete sample of 112 3CR radio galaxies with redshifts $z$ < 0.3, alongside a stellar mass matched control sample. Our results provide strong evidence for significant differences ($\sim$3$σ$) between the triggering mechanisms of the different sub-types of powerful radio AGN. The high-excitation radio galaxies (HERGs) show a high rate of morphological disturbance (62$^{+6}_{-7}$ per cent) -- an excess of $\sim$4$σ$ compared with the control sample -- consistent with them being predominantly triggered in galaxy mergers and interactions. In contrast, the low-excitation radio galaxies (LERGs) show a much lower rate of morphological disturbance (36$^{+7}_{-6}$ per cent), consistent with the control sample, and suggesting a different dominant triggering mechanism, such as the accretion of gas from the hot X-ray haloes of the host galaxies or galaxy clusters. We also demonstrate that, when considering the radio morphology, the FRII HERG sources preferentially reside in disturbed morphologies, a difference of $\sim$3$σ$ to the FRII LERG objects. This suggests that the FRII LERG sources do not solely represent a `switched-off' phase in the HERG lifecycle of the same parent galaxy population as the FRII HERGs.

Figures (17)

• Figure 1: Distributions of redshift, stellar mass, 1.4 GHz radio luminosity and [OIII]$\lambda$5007 emission-line luminosity for the HERGs (red) and the LERGs (blue) in the low-redshift ($z<0.05$) 3CR sample. Sources with no stellar mass estimates were not included in the corresponding plot. Those which only had upper limits on their [OIII]$\lambda$5007 emission-line luminosity were also not considered in the distribution.
• Figure 2: As in Figure \ref{['fig:low_z_3CR_distributions']}, but for the full 3CR sample ($z<0.3$) considered in this work.
• Figure 3: Stellar mass distribution of the 109 3CR sources which had stellar mass estimates, alongside the matched control sample distribution. The stellar mass distribution for the full 3CR sample, including those without control matches, is also shown.
• Figure 4: Examples of the 3CR galaxies classified as disturbed (top panel) and not disturbed (bottom panel) from the online interface classification (images for all the low-$z$ ($z<0.05$) are presented in Appendix \ref{['stamp_images']}). Each image is 200 kpc $\times$ 200 kpc, centred on the target galaxy, with 10 kpc scale bars to aid with classification (see Section \ref{['zooniverse']}). The high contrast (left of pair) and low contrast (right of pair) images are presented.
• Figure 5: Proportions of the whole 3CR sample which were classified as disturbed, not disturbed, or uncertain, alongside the matched control sample.