CANDELS: Constraining the AGN-Merger Connection with Host Morphologies at z~2

Abstract: Using HST/WFC3 imaging taken as part of the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS), we examine the role that major galaxy mergers play in triggering active galactic nuclei (AGN) activity at z~2. Our sample consists of 72 moderate-luminosity (Lx ~ 1E42-1E44 erg/s) AGN at 1.5<z<2.5 that are selected using the 4 Msec Chandra observations in the Chandra Deep Field South, the deepest X-ray observations to date. Employing visual classifications, we have analyzed the rest-frame optical morphologies of the AGN host galaxies and compared them to a mass-matched control sample of 216 non-active galaxies at the same redshift. We find that most of the AGN reside in disk galaxies (51.4%), while a smaller percentage are found in spheroids (27.8%). Roughly 16.7% of the AGN hosts have highly disturbed morphologies and appear to be involved in a major merger or interaction, while most of the hosts (55.6%) appear relatively relaxed and undisturbed. These fractions are statistically consistent with the fraction of control galaxies that show similar morphological disturbances. These results suggest that the hosts of moderate-luminosity AGN are no more likely to be involved in an ongoing merger or interaction relative to non-active galaxies of similar mass at z~2. The high disk fraction observed among the AGN hosts also appears to be at odds with predictions that merger-driven accretion should be the dominant AGN fueling mode at z~2, even at moderate X-ray luminosities. Although we cannot rule out that minor mergers are responsible for triggering these systems, the presence of a large population of relatively undisturbed disk-like hosts suggests that secular processes play a greater role in fueling AGN activity at z~2 than previously thought.

• The paper shows that a majority (51.4%) of AGN hosts are disk galaxies, challenging the view that major mergers are the primary trigger for AGN activity.
• It employs visual classification of 72 AGN and 216 control galaxies using high-resolution HST/WFC3 imaging and deep Chandra X-ray data to assess host morphology.
• The study finds no significant excess of merger-related disturbances in AGN hosts, suggesting that secular processes or minor interactions may fuel AGN at z~2.

Examination of AGN Host Morphologies and the AGN-Merger Connection at $z\sim2$

The paper presented by Kocevski et al. provides a detailed exploration of the morphological characteristics of active galactic nuclei (AGN) host galaxies at a significant redshift, $z\sim2$, using data from the Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS). This study aims to assess the role of major galaxy mergers in triggering AGN activity, leveraging high-resolution imaging from HST/WFC3 and comprehensive X-ray data from the Chandra Deep Field South.

Methodology

The authors employed a sample of 72 moderate-luminosity AGN, identified based on their X-ray characteristics, and compared their host morphologies to a control cohort of 216 non-active galaxies with matched redshift and mass parameters. Through visual classification, the research categorizes the galaxies into distinct morphological types and assesses the degree of disturbance suggestive of recent interactions or active merging processes.

Key Findings

1. High Disk Fraction: The study finds that a majority ($51.4^{+5.8}_{-5.9}\%$) of AGN hosts are disk-like galaxies, challenging the assumption that major mergers are the primary AGN-triggering mechanism. In comparison, spheroids account for $27.8^{+5.8}_{-4.6}\%$ of AGN hosts.
2. Morphological Comparisons: The researchers observed a higher inclination towards spheroid morphologies among AGN hosts than control galaxies ($27.8^{+5.8}_{-4.6}\%$ versus $16.9^{+2.8}_{-2.2}\%$). This underscores a deviation from simplistic merger-driven models, especially given the supposed prevalence of spheroids in merger interactions.
3. Interaction Frequency: There is no statistically significant excess of morphological disturbances among AGN hosts compared to the mass-matched control sample. Specifically, $16.7^{+5.3}_{-3.5}\%$ of AGN hosts are classified as undergoing significant merger activity, aligning closely with $15.5^{+2.8}_{-2.2}\%$ in the control group.
4. Luminosity Considerations: The study observes a stark morphological shift in the host properties of more luminous AGN ($L_{\rm X} > 10^{43}$ erg s$^{-1}$), which become predominantly spheroidal, hinting at a different fueling mechanism, possibly aligned with earlier merger events.

Implications and Future Directions

This work contributes vital evidence to the ongoing debate about the role of mergers in AGN activation. By establishing that most AGN are housed in relatively undisturbed disk galaxies, it suggests that secular processes or minor mergers could be the dominant mechanisms for AGN fueling at this epoch. Such findings question the detailed applicability of models that extensively prioritize major mergers as the primary catalyst for AGN phenomena at higher redshifts.

Future research should further quantify the contribution of these disk-hosted AGN to the X-ray luminosity function, which may illuminate the accurate prevalence of non-merger fueled AGN activity. Additionally, exploring the potential roles of other gravitational instabilities or rapid cold accretion processes could offer deeper insights into the diverse astrophysical pathways leading to AGN activation in the early Universe. These findings form a crucial step toward better understanding the complex interplay between galactic evolution and black hole growth.