Major Galaxy Mergers Only Trigger the Most Luminous AGN (1209.5393v1)

Abstract: Using multiwavelength surveys of active galactic nuclei across a wide range of bolometric luminosities (10^{43}<L_{bol}(erg/s<5x10^{46}) and redshifts (0<z<3), we find a strong, redshift-independent correlation between the AGN luminosity and the fraction of host galaxies undergoing a major merger. That is, only the most luminous AGN phases are connected to major mergers, while less luminous AGN appear to be driven by secular processes. Combining this trend with AGN luminosity functions to assess the overall cosmic growth of black holes, we find that ~50% by mass is associated with major mergers, while only 10% of AGN by number, the most luminous, are connected to these violent events. Our results suggest that to reach the highest AGN luminosities -where the most massive black holes accreted the bulk of their mass - a major merger appears to be required. The luminosity dependence of the fraction of AGN triggered by major mergers can successfully explain why the observed scatter in the M-\sigma relation for elliptical galaxies is significantly lower than in spirals. The lack of a significant redshift dependence of the L_{bol}-f_{merger} relation suggests that downsizing, i.e., the general decline in AGN and star formation activity with decreasing redshift, is driven by a decline in the frequency of major mergers combined with a decrease in the availability of gas at lower redshifts.

• The paper finds that only the most luminous AGN are associated with major mergers, with about 10% of AGN by number linked to these events.
• It uses multiwavelength surveys across 0 < z < 3 and bolometric luminosities between 10^43 and 10^46.5 erg s⁻¹ to correlate AGN luminosity with merger activity.
• The study highlights that major mergers drive high-accretion phases crucial for black hole growth, contributing nearly 50% of cosmic black hole mass buildup and dominating extragalactic UV output.

Major Galaxy Mergers and the Luminosity Dependence of AGN Triggering

The research paper by Treister et al. presents an in-depth analysis of the relationship between major galaxy mergers and the triggering of active galactic nuclei (AGN), with a particular focus on how this relationship varies with AGN luminosity. Using comprehensive multiwavelength surveys spanning a significant range of bolometric luminosities (43 < log(L_bol/erg s⁻¹) < 46.5) and redshifts (0 < z < 3), the authors explore the correlation between AGN luminosity and the fraction of host galaxies undergoing major mergers. This paper addresses a critical question in the field: what mechanisms trigger the episodic growth of supermassive black holes (SMBHs) during AGN phases?

Key Findings

1. Luminosity Dependence: The authors establish a strong correlation between AGN luminosity and the prevalence of major mergers among host galaxies. The analysis shows that only the most luminous AGN phases are associated with major mergers, while less luminous AGNs appear to be fueled by secular processes, e.g., minor interactions and instabilities within galaxies. Approximately 50% of the cosmic growth of black holes by mass is linked to major mergers, whereas only 10% of AGNs by number—the most luminous—are connected to these events.
2. Absence of Redshift Dependence: The paper finds that the relationship between AGN luminosity and merger fraction does not depend on redshift. This suggests that the observed downsizing in AGN and star formation activity is largely driven by changes in merger frequency and the availability of gas, rather than intrinsic evolution with cosmic time.
3. Contribution to Cosmic Backgrounds: Major mergers, while being significant instigators of luminous AGN activity, contribute roughly equally to the cosmic X-ray background as those triggered by secular processes. However, they dominate the extragalactic UV light due to their high luminosity and intensity of activity, which are critical in high-redshift environments.
4. Implications for Black Hole Growth: The dependence of black hole growth episodes on major mergers is notably significant, indicating that reaching the highest AGN luminosities requires a major merger event. Consequently, the paper suggests that major mergers are crucial during high-accretion phases needed for the development of the most massive black holes observed today.

Implications and Future Directions

The findings from Treister et al. have substantial implications for understanding the evolution of galaxies and SMBHs. By elucidating the conditions required for the most luminous AGN episodes, the paper provides insights into galaxy formation, AGN feedback processes, and the interplay between mergers and secular evolution. From a theoretical standpoint, these results can inform models of galaxy evolution that integrate the impact of AGN feedback on star formation histories and morphological transformations.

In terms of future research, the paper highlights the necessity of further investigations into the mechanics of AGN triggering across different scales and environments. The role of minor mergers and internal processes in lower luminosity AGNs remains an area for deeper exploration. Additionally, spectral and temporal analyses aiming to disentangle the life cycles of AGNs can provide further clarity on the dynamics of SMBH growth.

The research also stimulates interest in understanding the cosmic environment's evolution, particularly how changes in gas availability may mediate SMBH feeding mechanisms as the universe evolves. The potential integration of next-generation telescopes and surveys with high-resolution capabilities will be instrumental in mapping out the complex relationships identified in this paper, encouraging a refined understanding of galaxy and black hole co-evolution.