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

TL;DR

This study tests the long-standing question of whether galaxy mergers trigger AGN activity by analyzing 614 type $1$ AGNs at $z<0.07$ with tidal features identified in deep DESI Legacy Imaging Survey images, compared to a redshift- and $M_ ext{BH}$-matched inactive control sample. The authors quantify merger-triggering using $f_T$, the fraction of AGN hosts with tidal features, and its excess $e_T$ relative to controls, finding a strong, luminosity- and mass-dependent trend: $f_T$ rises from $0.05$ to $0.75$ as $\

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 $0.05\pm0.03$ to $0.75\pm0.13$ as the luminosity of the $[\mathrm{O\,{\scriptstyle III}}]$ $λ$5007 emission line ($L_{[\mathrm{O\,{\scriptstyle III}}]}$), an indicator for bolometric AGN luminosity, increases in the range $10^{39.5}\lesssim L_{[\mathrm{O\,{\scriptstyle III}}]}/(\mathrm{erg\,s}^{-1}) \lesssim10^{42.5}$. In addition, $f_T$ increases from $0.13\pm0.03$ to $0.43\pm0.09$ as black hole mass ($M_\mathrm{BH}$) increases in the range $10^{6.0}\lesssim M_\mathrm{BH}/M_{\odot}\lesssim10^{8.5}$. The fraction $f_T$ also increases with the Eddington ratio, although the trend is less significant compared to that with $L_{[\mathrm{O\,{\scriptstyle III}}]}$ and $M_\mathrm{BH}$. 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 $L_{[\mathrm{O\,{\scriptstyle III}}]}$ and weakly with $M_\mathrm{BH}$. 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.

Figures (10)

• Figure 1: Distributions of type 1 AGNs in the redshift vs. $\log M_\mathrm{BH}$ plane. Type 1 AGNs are indicated by colored circles, with the color representing $\log L_\mathrm{[O\,{\footnotesize III}]}$. See the color bar for the color scale.
• Figure 2: Histograms of the normalized distributions of redshift and $\log M_\mathrm{BH}$ (or equivalently, $\log M_\mathrm{bul}$) for the type 1 AGN sample and the matched inactive control sample.
• Figure 3: Examples of type 1 AGN hosts with tidal features, which have $\log(L_\mathrm{[O\,{\footnotesize III}]}$/erg s$^{-1})>41.4$. First row: Color images from SDSS. The galaxy ID is provided. The horizontal bar indicates the angular scale of the image. Second row: $r$-band deep images of the DESI Legacy Imaging Survey. The angular scale of the deep image matches that of the color image in the first row. Third row: Optical spectra of type 1 AGNs covering the observed wavelength range 4700-7400Å, in which H$\beta$, [O III], and H$\alpha$ emission lines are visible. Also shown are the values of $\log L_\mathrm{[O\,{\footnotesize III}]}$ and $\log M_\mathrm{BH}$, where $L_\mathrm{[O\,{\footnotesize III}]}$ is in units of erg s$^{-1}$ and $M_\mathrm{BH}$ is in units of solar mass ($M_{\odot}$).
• Figure 4: Same as Fig. 3 but for $41.1<\log(L_\mathrm{[O\,{\footnotesize III}]}$/erg s$^{-1})<41.5$.
• Figure 5: Same as Fig. 3 but for $40.4<\log(L_\mathrm{[O\,{\footnotesize III}]}$/erg s$^{-1})<41.1$.