Signatures of BH seeding on the $\mathrm{M_{\displaystyle \bullet}}-\sigma$ relation: Predictions from the BRAHMA simulations

TL;DR

This study uses BRAHMA simulations to explore how different black hole seeding models influence the evolution of the $ ext{M}_ullet- extsigma$ relation across cosmic time, providing insights into early black hole-galaxy coevolution.

Contribution

It introduces systematic variations in BH seeding models within simulations to predict their impact on the $ ext{M}_ullet- extsigma$ relation evolution.

Findings

Different seed models lead to distinct $ ext{M}_ullet- extsigma$ normalizations at high redshift.

More restrictive seed models show larger scatter and lower growth beyond seed mass.

The evolution of the relation depends on merger-driven and accretion-driven growth mechanisms.

Abstract

The James Webb Space Telescope (JWST) has identified a large population of supermassive ($10^6$-$10^8~\mathrm{M}_\odot$) black holes (BHs) in the early universe ($z \sim 4$-$7$). Current measurements suggest that many of these BHs exhibit higher BH-to-stellar mass ratios than local populations, opening a new window into the earliest stages of BH-galaxy coevolution and offering the potential to place tight constraints on BH seeding and growth in the early universe. In this work, we use the BRAHMA simulations to investigate the impact of BH seeding on the $\mathrm{M_{\bullet}}-\sigma$ relation. These simulations adopt heavy $\sim10^5~\mathrm{M}_{\odot}$ seeds and systematically varied BH seeding models, resulting in distinct predictions for seed abundances. We find that different seed models lead to different normalizations of the $\mathrm{M_{\bullet}}-\sigma$ relation at higher redshifts ($z > 2$) across all $\sigma$, and at low redshift for systems with low $\sigma$ ($50~\mathrm{km\ s^{-1}} \lesssim \sigma \lesssim 80~\mathrm{km\ s^{-1}}$). The most lenient seed model also shows negligible evolution in the $\mathrm{M_{\bullet}}-\sigma$ relation across redshift, while more restrictive models have substantially lower normalization on the $\mathrm{M_{\bullet}}-\sigma$ relation for high $\sigma$ ($\sim 100~\mathrm{km\ s^{-1}}$) at high redshifts, and evolve upward toward the local relation. We demonstrate that the $\mathrm{M_{\bullet}}-\sigma$ evolution is a direct consequence of merger-dominated BH growth in low mass galaxies ($\lesssim 10^9~M_{\odot}$) and accretion dominated BH growth in high mass ($\gtrsim10^9~M_{\odot}$) galaxies. Furthermore, the scatter in the $\mathrm{M_{\bullet}}-\sigma$ relation is larger for the more restrictive models due to the inability of many BHs to grow significantly beyond their seed mass.