On the orientation and magnitude of the black hole spin in galactic nuclei

TL;DR

This paper investigates how different degrees of anisotropic accretion influence the evolution of black hole spin magnitude and orientation in galactic nuclei, revealing two distinct growth regimes and their observational implications.

Contribution

It introduces a model considering varying anisotropy levels in accretion, moving beyond the traditional chaotic or coherent assumptions, and explores their effects on black hole spin evolution.

Findings

Early phase shows rapid spin alignment with large spins (~0.8).

Massive black holes can spin up to ~1 with modest anisotropy.

Spin direction remains stable with anisotropic fueling.

Abstract

Massive black holes in galactic nuclei vary their mass M and spin vector J due to accretion. In this study we relax, for the first time, the assumption that accretion can be either chaotic, i.e. when the accretion episodes are randomly and isotropically oriented, or coherent, i.e. when they occur all in a preferred plane. Instead, we consider different degrees of anisotropy in the fueling, never confining to accretion events on a fixed direction. We follow the black hole growth evolving contemporarily mass, spin modulus a and spin direction. We discover the occurrence of two regimes. An early phase (M <~ 10 million solar masses) in which rapid alignment of the black hole spin direction to the disk angular momentum in each single episode leads to erratic changes in the black hole spin orientation and at the same time to large spins (a ~ 0.8). A second phase starts when the black hole mass increases above >~ 10 million solar masses and the accretion disks carry less mass and angular momentum relatively to the hole. In the absence of a preferential direction the black holes tend to spin-down in this phase. However, when a modest degree of anisotropy in the fueling process (still far from being coherent) is present, the black hole spin can increase up to a ~ 1 for very massive black holes (M >~ 100 million solar masses), and its direction is stable over the many accretion cycles. We discuss the implications that our results have in the realm of the observations of black hole spin and jet orientations.