Direct observational evidence that higher-luminosity type 1 active galactic nuclei are most commonly triggered by galaxy mergers

¹ Department of Astronomy and Atmospheric Sciences, College of Natural Sciences, Kyungpook National University, Daegu 41566, Republic of Korea
² School of Liberal Studies, Sejong University, 209 Neungdong-ro, Gwangjin-Gu, Seoul 05006, Republic of Korea
³ Department of Physics and Astronomy, Sejong University, 209 Neungdong-ro, Gwangjin-Gu, Seoul 05006, Republic of Korea
⁴ Department of Earth Sciences, Pusan National University, Busan 46241, Republic of Korea
⁵ Korea Astronomy and Space Science Institute (KASI), 776 Daedeokdae-ro, Yuseong-gu, Daejeon 34055, Republic of Korea

^★ Corresponding authors: yyoon@knu.ac.kr; yongjungkim@sejong.ac.kr; dh.dr2kim@gmail.com.

Received: 23 April 2025
Accepted: 24 October 2025

Abstract

We examine the connection between galaxy mergers and the triggering of active galactic nuclei (AGNs) using a sample of 614 type 1 AGNs at z < 0.07, along with a control sample of inactive galaxies matched to the AGNs for comparison. We used tidal features, detected in deep images from the DESI Legacy Imaging Survey, as direct evidence of recent mergers. We find that the fraction of type 1 AGN hosts with tidal features (f_T) is higher for AGNs with higher luminosities and (to a lesser extent) more massive black holes. Specifically, f_T rapidly increases from 0.05 ± 0.03 to 0.75 ± 0.13 as the luminosity of the [O III] λ5007 emission line (L_[O III]), an indicator for bolometric AGN luminosity, increases in the range 10^39.5 ≲ L_[O III]/(erg s⁻¹) ≲ 10^42.5. In addition, f_T increases from 0.13 ± 0.03 to 0.43 ± 0.09 as black hole mass (M_BH) increases in the range 10^6.0 ≲ M_BH/M_⊙ ≲ 10^8.5. The fraction f_T also increases with the Eddington ratio, although the trend is less significant compared to that with L_[O III] and M_BH. The excess of f_T, defined as the ratio of f_T for AGNs to that of their matched inactive counterparts, exhibits similar trends, primarily increasing with L_[O III] and weakly with M_BH. Our results indicate that, in the local Universe, galaxy mergers are the predominant triggering mechanism for high-luminosity AGNs, whereas they play a lesser role in triggering lower-luminosity AGNs. Additionally, strong events, such as galaxy mergers, may be more necessary to activate massive black holes in more massive galaxies due to their lower gas fractions.