Black-Hole Spin and Galactic Morphology

TL;DR

This paper explores how the differences in black hole spins in elliptical and spiral galaxies can explain the observed bimodality in AGN radio-loudness, linking galaxy morphology to black hole evolution.

Contribution

It demonstrates that galaxy morphology influences black hole spin evolution through accretion processes, supporting the link between galaxy type and AGN properties.

Findings

Elliptical galaxy black holes tend to have higher spins due to merger-driven accretion.

Spiral galaxy black holes have lower spins, affected by small, random accretion episodes.

The results support the conjecture linking black hole spin bimodality to galaxy morphology.

Abstract

We investigate the conjecture by Sikora, Stawarz & Lasota (2007) that the observed AGN--radio-loudness bimodality can be explained by the morphology-related bimodality of black-hole spin distribution in the centers of galaxies: central black holes in giant elliptical galaxies may have (on average) much larger spins than black holes in spiral/disc galaxies. We study how accretion from a warped disc influences the evolution of black hole spins and conclude that within the cosmological framework, where the most massive BHs have grown in mass via merger driven accretion, one indeed expects most supermassive black holes in elliptical galaxies to have on average higher spin than black holes in spiral galaxies, where random, small accretion episodes (e.g. tidally disrupted stars, accretion of molecular clouds) might have played a more important role.