Supermassive black holes in disk-dominated galaxies outgrow their bulges and co-evolve with their host galaxies

Abstract: The deep connection between galaxies and their supermassive black holes is central to modern astrophysics and cosmology. The observed correlation between galaxy and black hole mass is usually attributed to the contribution of major mergers to both. We make use of a sample of galaxies whose disk-dominated morphologies indicate a major-merger-free history and show that such systems are capable of growing supermassive black holes at rates similar to quasars. Comparing black hole masses to conservative upper limits on bulge masses, we show that the black holes in the sample are typically larger than expected if processes creating bulges are also the primary driver of black hole growth. The same relation between black hole and total stellar mass of the galaxy is found for the merger-free sample as for a sample which has experienced substantial mergers, indicating that major mergers do not play a significant role in controlling the coevolution of galaxies and black holes. We suggest that more fundamental processes which contribute to galaxy assembly are also responsible for black hole growth.

• The paper finds that SMBHs in disk-dominated galaxies grow at quasar-like rates despite minimal bulge development.
• The paper uses broad Hα emission analysis to estimate black hole masses from 2×10⁶ to 9×10⁸ M☉ in a carefully selected sample of 101 spiral galaxies.
• The paper proposes that secular processes, not only mergers, play a significant role in the co-evolution of supermassive black holes and their host galaxies.

Co-evolution of Supermassive Black Holes and Disk-Dominated Galaxies

This paper explores the relationship between supermassive black holes (SMBHs) and their host galaxies, specifically focusing on disk-dominated galaxies that have not undergone recent major mergers. Traditionally, the correlation between black hole mass and galaxy mass has been attributed to the effects of galaxy mergers, which are believed to grow both stellar bulges and central black holes. This study seeks to understand if this correlation holds true in galaxies largely free from such merger-driven processes.

Research Highlights

• Sample Selection: The authors select a sample of 101 disk-dominated galaxies with visible spiral structures and unobscured AGNs. These galaxies, spanning redshifts below 0.25, display clear evidence of active black hole growth. The sample was selected to exclude those with major merger-induced bulge growth, thereby focusing on alternate, possibly secular, paths to black hole and galaxy co-evolution.
• Black Hole Masses and Growth Rates: By analyzing broad $\text{H}\alpha$ line emission, the study estimates black hole masses between $2 \times 10^6$ and $9 \times 10^8~M_\odot$. Additionally, accretion rates suggest that these black holes can grow at rates comparable to quasars, despite the absence of significant mergers.
• Black Hole-Bulge Relation: When analyzing the mass correlation between black holes and bulges in these disk-dominated galaxies, the study finds a lack of significant relationship. This is in contrast to findings in bulge-dominated, merger-rich galaxies, where a strong correlation exists.
• Black Hole-Total Stellar Mass Relation: For both the disk-dominated galaxies in this study and the comparison sample of bulge-dominated galaxies from prior research, the correlation between black hole mass and total stellar mass holds no significant distinction. This implies that the processes fostering SMBH growth and thus the co-evolution of black holes and galaxies may not be exclusively merger-driven.

Implications

The paper suggests a fundamental mechanism governing the relationship between black holes and their host galaxies that operates independently of merger-driven processes. While mergers clearly impact galaxy structure and dynamics, this research implies additional mechanisms—such as secular processes or micro-accumulations of mass—that also consistently lead to the observed correlations. This suggests a potentially universal aspect of galaxy evolution, which may be rooted in underlying gravitational potentials or AGN feedback mechanisms that regulate both black hole growth and stellar assembly.

Speculation on Future Developments

In advancing this understanding of galaxy-black hole co-evolution, future investigations would benefit from:

1. High-Resolution Observations: Employing instruments capable of accurately measuring stellar dynamics and black hole masses in low-bulge galaxies.
2. Simulations of Secular Evolution: Incorporating models with refined secular processes and cold accretion scenarios to discern how these complicitly mimic or differ from merger-induced growth.
3. Comparative Studies Across Morphologies: Deploying observational studies and simulations to test whether the black hole-total stellar mass relation can be scaled across the broader population of galaxies, including dwarf and irregular morphological types.

This research positions itself at the intersection of galaxy morphology and black hole astrophysics, challenging prevailing paradigms about the drivers of SMBH growth and suggesting the need to reconsider aspects of large-scale galaxy evolution theories.