Causal Discovery in Astrophysics: Unraveling Supermassive Black Hole and Galaxy Coevolution

TL;DR

This paper applies advanced causal discovery methods to astrophysical data, revealing different causal relationships between supermassive black holes and galaxy properties depending on galaxy morphology, thus providing new insights into galaxy evolution.

Contribution

It introduces a Bayesian causal discovery framework to astrophysics, uncovering causal directions between SMBH masses and galaxy features, with implications for galaxy formation theories.

Findings

In elliptical galaxies, bulge properties influence SMBH growth.

In spiral galaxies, SMBHs affect host galaxy properties.

Causal relationships differ by galaxy morphology, supporting hierarchical galaxy assembly models.

Abstract

Correlation does not imply causation, but patterns of statistical association between variables can be exploited to infer a causal structure (even with purely observational data) with the burgeoning field of causal discovery. As a purely observational science, astrophysics has much to gain by exploiting these new methods. The supermassive black hole (SMBH)--galaxy interaction has long been constrained by observed scaling relations, that is low-scatter correlations between variables such as SMBH mass and the central velocity dispersion of stars in a host galaxy's bulge. This study, using advanced causal discovery techniques and an up-to-date dataset, reveals a causal link between galaxy properties and dynamically-measured SMBH masses. We apply a score-based Bayesian framework to compute the exact conditional probabilities of every causal structure that could possibly describe our galaxy sample. With the exact posterior distribution, we determine the most likely causal structures and notice a probable causal reversal when separating galaxies by morphology. In elliptical galaxies, bulge properties (built from major mergers) tend to influence SMBH growth, while in spiral galaxies, SMBHs are seen to affect host galaxy properties, potentially through feedback in gas-rich environments. For spiral galaxies, SMBHs progressively quench star formation, whereas in elliptical galaxies, quenching is complete, and the causal connection has reversed. Our findings support theoretical models of hierarchical assembly of galaxies and active galactic nuclei feedback regulating galaxy evolution. Our study suggests the potentiality for further exploration of causal links in astrophysical and cosmological scaling relations, as well as any other observational science.