Evidence for non-merger co-evolution of galaxies and their supermassive black holes

Abstract: Recent observational and theoretical studies have suggested that supermassive black holes (SMBHs) grow mostly through non-merger (`secular') processes. Since galaxy mergers lead to dynamical bulge growth, the only way to observationally isolate non-merger growth is to study galaxies with low bulge-to-total mass ratio (e.g. B/T < 10%). However, bulge growth can also occur due to secular processes, such as disk instabilities, making disk-dominated selections a somewhat incomplete way to select merger-free systems. Here we use the Horizon-AGN simulation to select simulated galaxies which have not undergone a merger since z = 2, regardless of bulge mass, and investigate their location on typical black hole-galaxy scaling relations in comparison to galaxies with merger dominated histories. While the existence of these correlations has long been interpreted as co-evolution of galaxies and their SMBHs driven by galaxy mergers, we show here that they persist even in the absence of mergers. We find that the correlations between SMBH mass and both total mass and stellar velocity dispersion are independent of B/T ratio for both merger-free and merger-dominated galaxies. In addition, the bulge mass and SMBH mass correlation is still apparent for merger-free galaxies, the intercept for which is dependent on B/T. Galaxy mergers reduce the scatter around the scaling relations, with merger-free systems showing broader scatter. We show that for merger-free galaxies, the co-evolution is dominated by radio-mode feedback, and suggest that the long periods of time between galaxy mergers make an important contribution to the co-evolution between galaxies and SMBHs in all galaxies.

• The paper demonstrates that key scaling relations between SMBH mass and galaxy properties persist even in the absence of mergers, underscoring the role of secular evolution.
• The research finds that the bulge-to-total mass ratio affects the SMBH–bulge mass correlation but not the relation with total stellar mass, hinting at distinct co-evolution pathways.
• The study reveals that while mergers reduce scatter in scaling relations, sustained jet-mode feedback in merger-free systems actively regulates AGN activity and star formation.

Evidence for Non-Merger Co-Evolution of Galaxies and Their Supermassive Black Holes

This paper provides a comprehensive analysis of the co-evolution of galaxies and their central supermassive black holes (SMBHs), particularly emphasizing the mechanisms outside of galaxy mergers. The analysis utilizes the Horizon-AGN simulation to explore galaxy-SMBH scaling relations, focusing on systems that have evolved without significant mergers since redshift $z=2$.

Key Findings

1. Persistence of Scaling Relations in Merger-Free Galaxies: The paper demonstrates that traditional scaling relations, such as those between SMBH mass and galaxy properties (e.g., total stellar mass, bulge mass, stellar velocity dispersion), persist even in the absence of mergers. This suggests a reconsideration of the role of non-merger processes, such as secular evolution, in driving these correlations.
2. Influence of Bulge-to-Total Mass Ratio: It reveals that for merger-free systems, the bulge-to-total (B/T) mass ratio is a determinant of the intercept in the SMBH-bulge mass relation. However, this ratio shows no significant impact on the correlation between SMBH and total stellar mass, supporting the hypothesis of independent co-evolution pathways.
3. Mergers as Scatter Reducers: The study shows that mergers reduce scatter in the scaling relations and strengthen these correlations' slopes. This suggests that mergers have an averaging effect that tightens these relations, particularly for the SMBH total stellar mass correlation.
4. AGN Luminosity and Feedback Modes: By examining AGN luminosity, the paper finds no significant difference between merger-dominated and merger-free systems when matched for SMBH mass at low redshift. The authors propose that these systems may show similarities due to jet-mode feedback dominating the AGN activity for extended periods.
5. Role of Jet/Radio Feedback: The secular co-evolution may be driven by jet/radio mode feedback, which dominates the time SMBHs spend in active phases rather than the quasar mode. This finding points toward an important role for low-luminosity AGN feedback in regulating star formation and SMBH growth in the absence of mergers.

Implications

The results have significant implications for understanding galaxy evolution that deviate from traditional paradigms emphasizing mergers. By highlighting how secular processes can independently foster SMBH-galaxy co-evolution across a significant portion of the cosmic history, this research sets the stage for further investigation into the mechanisms of SMBH growth and galaxy formation.

The broader scatter observed in non-merger systems could suggest variation across different cosmological scenarios, implying larger diversity in galaxy and SMBH evolution pathways than previously accounted for. These findings resonate with observations of SMBH in so-called "bulgeless" galaxies more massive than predicted, supporting a framework where galaxy and SMBH can co-evolve without frequent mergers.

Future Research Directions

The study opens several avenues for future inquiry. Observations utilizing high-resolution IFU instruments could test these predictions by examining merger-free systems' star formation and AGN activity. Furthermore, detailed studies of AGN feedback's role, particularly in jet mode, within different simulation environments could deepen our understanding of how such processes drive co-evolution in both merger and non-merger dominated systems.

The evidence presented herein challenges traditional views and suggests a more nuanced narrative where internal processes play a pivotal role in shaping the complex relation between galaxies and their central black holes. This augurs well for a deeper integration of secular evolution processes in theoretical models and simulations within astrophysics.