Coevolution (Or Not) of Supermassive Black Holes and Host Galaxies

Abstract: We review the observed demographics and inferred evolution of supermassive black holes (BHs) found by dynamical modeling of spatially resolved kinematics. Most influential was the discovery of a tight correlation between BH mass and the velocity dispersion of the host-galaxy bulge. It and other correlations led to the belief that BHs and bulges coevolve by regulating each other's growth. New results are now replacing this simple story with a richer and more plausible picture in which BHs correlate differently with different galaxy components. BHs are found in pure-disk galaxies, so classical (elliptical-galaxy-like) bulges are not necessary to grow BHs. But BHs do not correlate with galaxy disks. And any correlations with disk-grown pseudobulges or halo dark matter are so weak as to imply no close coevolution. We suggest that there are four regimes of BH feedback. 1- Local, stochastic feeding of small BHs in mainly bulgeless galaxies involves too little energy to result in coevolution. 2- Global feeding in major, wet galaxy mergers grows giant BHs in short, quasar-like "AGN" events whose feedback does affect galaxies. This makes classical bulges and coreless-rotating ellipticals. 3- At the highest BH masses, maintenance-mode feedback into X-ray gas has the negative effect of helping to keep baryons locked up in hot gas. This happens in giant, core-nonrotating ellipticals. They inherit coevolution magic from smaller progenitors. 4- Independent of any feedback physics, the averaging that results from successive mergers helps to engineer tight BH correlations.

• The paper demonstrates that supermassive black holes can form without classical bulges, challenging traditional coevolution models.
• It reveals significantly weaker correlations between black hole mass and disk or pseudobulge components compared to classical bulges.
• The study finds that mergers yield unexpectedly low black hole masses while highlighting a tight link between globular clusters and black hole growth.

Insights into the Coevolution (or Lack Thereof) of Supermassive Black Holes and Their Host Galaxies

The paper by Kormendy and Ho explores the nuanced relationship between supermassive black holes (BHs) and the galaxies that host them. It challenges the previously held assumption that BHs and their host galaxies coevolve tightly, suggesting instead a more complex interaction influenced by galaxy morphology and merger history.

Key Findings

1. Diverse BH-Galaxy Relationships: The research highlights that BHs do not strictly require a classical bulge for their formation, contradicting earlier assumptions. Significant examples are BHs found in bulgeless galaxies, indicating that classical bulges are not a prerequisite for BH formation.
2. Weak Correlations with Disks and Pseudobulges: While BHs exhibit strong correlations with classical bulges and elliptical galaxies, the paper demonstrates that such correlations are notably weaker with disk components and pseudobulges. This distinction underscores the different evolutionary pathways of galaxies, with pseudobulges forming through secular processes rather than major mergers.
3. The Impact of Mergers: The study reveals an intriguing insight into ongoing mergers, where these systems tend to host BHs with masses smaller than expected given their galaxy luminosities and velocity dispersions. This suggests that the immediate product of a merger does not always follow the established BH-scaling relations, likely due to complex dynamics during such events.
4. Insights from Globular Clusters: An interesting correlation between the total number of globular clusters (N_GC) and the BH mass is noted, with a tight relation observed in classical bulges and elliptical galaxies. This correlation implies a shared evolutionary history potentially tied to the galaxy's assembly processes.
5. Dark Matter and Bulge Correlations: Crucially, the study posits that there is no special coevolutionary relationship between BHs and dark matter halos that surpasses the relationship with visible matter in bulges. This finding challenges theories suggesting that dark matter dynamics play a direct, fundamental role in BH growth beyond providing a gravitational framework.

Implications and Future Directions

This paper's findings prompt a reevaluation of the mechanisms driving BH and galaxy growth. The weak correlation between BHs and pseudobulges, alongside their presence in bulgeless galaxies, suggests alternate growth pathways possibly dominated by local processes rather than the more global, merger-driven processes that characterize classical bulge evolution.

These results have profound implications for the theoretical modeling of galaxy formation and evolution, emphasizing the need to consider varied evolutionary tracks rather than single, universal models. Furthermore, the implications extend to cosmological simulations where the role of BHs in feedback processes must be reconsidered, especially in light of the diversity in BH-galaxy scaling relations observed across different galaxy morphologies.

Kormendy and Ho's work invites further observational and theoretical endeavors to refine our understanding of BH growth mechanisms and their intricate ties to galaxy evolution. As techniques in astronomical measurement continue to refine, and as more large-scale surveys come online, we anticipate a deeper understanding of the interplay between supermassive black holes, their immediate stellar environments, and the broader galactic context. This nuanced view is essential for unraveling the broader narrative of structure formation in the universe.