Black Hole-Galaxy Correlations without Self-Regulation

TL;DR

This study challenges the necessity of self-regulation in black hole growth models, proposing an alternative torque-limited accretion model that aligns with observed galaxy-black hole scaling relations without requiring feedback-driven self-regulation.

Contribution

The paper introduces a torque-limited accretion model for black hole growth that does not depend on self-regulation, contrasting with traditional feedback-based models.

Findings

Torque-limited model reproduces observed scaling relations.

Black hole growth correlates with galaxy disk feeding, not mergers.

Model allows rapid early black hole growth, consistent with observations.

Abstract

Recent models of black hole growth in a cosmological context have forwarded a paradigm in which the growth is self-regulated by feedback from the black hole itself. Here we use cosmological zoom simulations of galaxy formation down to z = 2 to show that such strong self-regulation is required in the popular spherical Bondi accretion model, but that a plausible alternative model in which black hole growth is limited by galaxy-scale torques does not require self-regulation. Instead, this torque-limited accretion model yields black holes and galaxies evolving on average along the observed scaling relations by relying only on a fixed, 5% mass retention rate onto the black hole from the radius at which the accretion flow is fed. Feedback from the black hole may (and likely does) occur, but does not need to couple to galaxy-scale gas in order to regulate black hole growth. We show that this result is insensitive to variations in the initial black hole mass, stellar feedback, or other implementation details. The torque-limited model allows for high accretion rates at very early epochs (unlike the Bondi case), which if viable can help explain the rapid early growth of black holes, while by z = 2 it yields Eddington factors of 1%-10%. This model also yields a less direct correspondence between major merger events and rapid phases of black hole growth. Instead, growth is more closely tied to cosmological disk feeding, which may help explain observational studies showing that, at least at z > 1, active galaxies do not preferentially show merger signatures.