The bulk of the black hole growth since z~1 occurs in a secular universe: No major merger-AGN connection

Abstract: What is the relevance of major mergers and interactions as triggering mechanisms for active galactic nuclei (AGN) activity? To answer this longstanding question, we analyze 140 XMM-selected AGN host galaxies and a matched control sample of 1264 inactive galaxies over z~0.3-1.0 and log(M_*/M_sun)<11.7 with high-resolution HST/ACS imaging from the COSMOS field. The visual analysis of their morphologies by 10 independent human classifiers yields a measure of the fraction of distorted morphologies in the AGN and control samples, i.e. quantifying the signature of recent mergers which might potentially be responsible for fueling/triggering the AGN. We find that (1) the vast majority (>85%) of the AGN host galaxies do not show strong distortions, and (2) there is no significant difference in the distortion fractions between active and inactive galaxies. Our findings provide the best direct evidence that, since z~1, the bulk of black hole accretion has not been triggered by major galaxy mergers, therefore arguing that the alternative mechanisms, i.e., secular processes and minor interactions, are the leading triggers for the episodes of major black hole growth. We also exclude an alternative interpretation of our results: a significant time lag between merging and the observability of the AGN phase could wash out the most significant merging signatures, explaining the lack of enhancement of strong distortions on the AGN hosts. We show that this alternative scenario is unlikely due to: (1) recent major mergers being ruled out for the majority of sources due to the high fraction of disk-hosted AGN, (2) the lack of a significant X-ray signal in merging inactive galaxies as a signature of a potential buried AGN, and (3) the low levels of soft X-ray obscuration for AGN hosted by interacting galaxies, in contrast to model predictions.

• The paper finds that over 85% of AGN host galaxies show no strong merger features, emphasizing secular processes over major mergers.
• It utilizes visual morphological classification of 140 X-ray-selected AGNs and 1264 inactive galaxies from HST/ACS COSMOS data.
• X-ray stacking and distortion comparisons highlight that SMBH growth since z~1 is driven more by disk dynamics than merging events.

Overview of "The Bulk of the Black Hole Growth Since z~1 Occurs in a Secular Universe: No Major Merger-AGN Connection"

This essay provides a detailed analysis of the research conducted by Mauricio Cisternas et al., exploring the role of major mergers in the growth of supermassive black holes (SMBHs) since the redshift of approximately z~1. The paper evaluated the morphology of active galactic nucleus (AGN) host galaxies to understand the significance of major mergers in AGN triggering and black hole growth, using a substantial sample from the COSMOS survey.

Methodology

The study utilized data from 140 X-ray-selected AGNs and a control set of 1264 inactive galaxies, both observed using the HST/ACS in the COSMOS field. A key approach was the visual analysis of galaxy morphologies by 10 independent classifiers, assessing the extent of distortion as evidence of past mergers. This classification allowed the researchers to compare the incidence of merger signatures between AGN hosts and inactive galaxies.

Key Findings

1. Lack of Strong Distortions in AGN Hosts: The majority of AGN host galaxies do not exhibit significant morphological distortions, indicating a lack of recent major mergers. Over 85% of AGN hosts lack strong distortion features.
2. Comparable Distortion Rates: No significant differences in distortion fractions between AGN host galaxies and the control sample of inactive galaxies were observed. This suggests that major mergers have not predominantly fueled AGN activity since z~1.
3. Implications of Disk-Dominance: A significant fraction of AGN host galaxies are disk-dominated, indicating that secular processes or minor interactions—rather than major mergers—are more likely to drive black hole growth in these epochs.
4. Alternative Explanations Disfavored: The research addressed and challenged potential alternative explanations, such as the scenario where merging and AGN activity are temporally misaligned, proposing that AGN visibility is delayed after a merger. The data showed no compelling support for this hypothesis.
5. X-ray Stacking Analysis: A stacking analysis of inactive galaxies with potential merger activity revealed no clear X-ray signals indicative of obscured AGN, supporting the argument that ongoing mergers are not significantly contributing to black hole accretion.

Implications

These findings challenge conventional models that view major mergers as a leading trigger for AGN activity and SMBH growth in the proposed time frame. The absence of a notable difference in merger signatures between AGNs and inactive galaxies suggests that alternative fueling mechanisms, like internal secular processes and minor mergers, dominate since z~1. This shift introduces a need for updated models of galaxy evolution that account for these mechanisms more prominently.

Future Directions

Future research is encouraged to further explore the role of secular processes in SMBH growth and to rigorously examine potential minor merger contributions. Additionally, extending these studies to higher redshifts might offer insights into whether the dominance of non-merger-driven processes in black hole growth observed since z~1 holds at earlier cosmic times.

In conclusion, the research carried out by Cisternas et al. provides compelling evidence that, contrary to some prevailing theories, major mergers do not significantly fuel the majority of AGN activity since z~1. This insight into the mechanisms behind SMBH growth calls for a reevaluation of models regarding galaxy evolution and black hole accretion dynamics.