# Black Hole -- Galaxy Correlations in Simba

## TL;DR

This study uses the Simba simulation to explore the co-evolution of galaxies and supermassive black holes, revealing how black hole growth, accretion modes, and galaxy properties are interconnected over cosmic time.

## Contribution

It introduces a detailed simulation model for black hole growth and feedback, providing new insights into their co-evolution with galaxies across different epochs.

## Key findings

- Black hole mass--stellar velocity dispersion relation matches observations.
- Black hole accretion rate correlates with star formation rate.
- Black hole properties depend on galaxy gas content and star formation.

## Abstract

We examine the co-evolution of galaxies and supermassive black holes in the Simba cosmological hydrodynamic simulation. Simba grows black holes via gravitational torque-limited accretion from cold gas and Bondi accretion from hot gas, while feedback from black holes is modeled in radiative and jet modes depending on the Eddington ratio ($f_{Edd}$). Simba shows generally good agreement with local studies of black hole properties, such as the black hole mass--stellar velocity dispersion ($M_{BH}-\sigma$) relation, 2 the black hole accretion rate vs. star formation rate (BHAR--SFR), and the black hole mass function. $M_{BH}-\sigma$ evolves such that galaxies at a given $M_{BH}$ have higher $\sigma$ at higher redshift, consistent with no evolution in $M_{BH}-M_*$. For $M_{BH}<\sim 10^8 M_\odot$, $f_{Edd}$ is anti-correlated with $M_{BH}$ since the BHAR is approximately independent of $M_{BH}$, while at higher masses $f_{Edd}-M_{BH}$ flattens and has a larger scatter. BHAR vs. SFR is invariant with redshift, but $f_{Edd}$ drops steadily with time at a given $M_{BH}$, such that all but the most massive black holes are accreting in a radiatively efficient mode at $z>\sim 2$. The black hole mass function amplitude decreases with redshift and is locally dominated by quiescent galaxies for $M_{BH}>10^{8}M_{\odot}$, but for $z>\sim 1$ star forming galaxies dominate at all $M_{BH}$. The $z=0$ $f_{Edd}$ distribution is roughly lognormal with a peak at $f_{Edd}<\sim 0.01$ as observed, shifting to higher $f_{Edd}$ at higher redshifts. Finally, we study the dependence of black hole properties with \HI\ content and find that the correlation between gas content and star formation rate is modulated by black hole properties, such that higher SFR galaxies at a given gas content have smaller black holes with higher $f_{Edd}$