Tracing the AGN-Merger Connection: insights from cosmological simulations and JWST mock observations

Abstract

Galaxy mergers have long been proposed as a mechanism for funneling gas toward galactic centres, potentially triggering accretion onto supermassive black holes (SMBHs) and igniting active galactic nuclei (AGN). While simulations often support this scenario, observational studies have yielded conflicting results regarding the AGN-merger connection. In this study, we analyze 31 galaxies from cosmological zoom-in simulations spanning redshifts $0.5 < z < 3$. We identify mergers using detailed merger trees based on six-dimensional dark matter particle information and identify AGN activity through SMBH accretion histories. To bridge the gap between simulations and observations, we generate mock JWST-like images and extract non-parametric morphological parameters. Employing a $k$-nearest neighbours (KNN) classifier in a five-dimensional space (four morphological parameters and redshift), we identify mergers in the mock-observed dataset. Our analysis reveals a statistically significant enhancement of AGN activity in merging systems, particularly at lower redshifts ($0.5 < z < 0.9$), where central gas reservoirs are more depleted. This supports the view that mergers contribute more significantly to AGN triggering in environments with low internal gas reservoirs, while their impact may be less pronounced in gas-rich systems. However, when relying solely on morphological classifications from mock observations, the observed AGN-merger connection weakens, especially at higher redshifts. This underscores the challenges in detecting merger-induced AGN activity observationally and highlights the importance of combining simulations with realistic mock observations to fully understand the AGN-merger relationship.

Figures (12)

• Figure 1: Redshift evolution of galaxy merger fractions under different merger definitions. The error bars denote the 68% Bayesian credible intervals computed from the quantiles of the beta distribution Cameron2011. We test eight definitions that vary in (i) stellar mass ratio thresholds (major vs. major+minor; left vs. right), (ii) time window symmetry (post-merger only vs. pre+post-merger; upper vs. lower), and (iii) time window width (0.3 Gyr vs. 0.5 Gyr; blue vs. red). Observational results include visual classifications from Ren2023 (black dashed lines) and close-pair statistics from Ventou2017 (gray dot-dashed). OLeary2021 (gray dashed) shows merger fractions derived from an empirical model.
• Figure 2: Redshift evolution of the bolometric luminosities of central SMBHs for two example galaxies. The instantaneous luminosity measured at each simulation snapshot is shown as dashed lines, while the maximum luminosity within the preceding $10^5\,\rm yr$ is shown as solid lines. Snapshots classified as AGN based on whether the luminosity exceeded the threshold at any point within the past $10^5\,\rm yr$ are marked with red circles. Gray vertical dashed lines indicate cases where AGN activity would be missed if only the instantaneous luminosity were considered, but is recovered using the past-activity-based definition.
• Figure 3: Intrinsic connection between galaxy mergers and AGN activity across redshift. The upper panel shows the AGN fraction among merging galaxies (solid green) compared to their non-merging control counterparts (dashed green), and the merger fraction among AGNs (solid purple) compared to non-AGN controls (dashed purple). The lower panel presents the inverse enhancement ratio ($1/R$) for both cases, along with corresponding error bars. Although most observational studies present the enhancement in terms of $R$, we use $1/R$ to avoid divergence issues when the control sample fractions are small. For convenience, the equivalent $R$ values are also indicated on the right-hand $y$-axis, inverted for clarity.
• Figure 4: Cold ($<10^4\,\rm K$) gas fractions (within $<0.1\,R_{\rm vir}$) relative to the combined mass of cold gas and stars for all galaxies in our sample. Purple symbols denote AGN hosts and grey symbols non-AGN galaxies. Mergers and non-mergers are distinguished by circles and crosses, respectively. The solid line represents the median value at each redshift bin.
• Figure 5: Workflow for generating realistic mock JWST galaxy images. We first create pristine radiative transfer images using powderday (left). Surface brightness dimming is applied according to the cosmological scaling $(1+z)^{-4}$, followed by convolution with the JWST/NIRCam F277W point spread function (middle). A randomly selected $512 \times 512$ patch from the CEERS survey is injected as a background to produce the final mock observation (right). After SB dimming, the brightest pixel values correspond to $\sim 10\,\rm nJy$.