Evolution of massive black hole spins

TL;DR

This paper reviews how the spins of massive black holes evolve over cosmic time, influenced by mergers and accretion, and explores their impact on quasar emissions and black hole retention in galaxies.

Contribution

It provides a comprehensive overview of recent simulations and theories on black hole spin evolution, highlighting the effects of mergers and accretion on spin dynamics.

Findings

Black hole spins influence quasar radiative efficiency.

Mergers can cause gravitational recoil affecting black hole retention.

Accretion processes significantly alter black hole spins over time.

Abstract

Black hole spins affect the efficiency of the "classical" accretion processes, hence the radiative output from quasars. Spins also determine how much energy is extractable from the hole itself. Recently it became clear that massive black hole spins also affect the retention of black holes in galaxies, be cause of the impulsive "gravitational recoil", up to thousands km/s, due to anisotropic emission of gravitational waves at merger. I discuss here the evolution of massive black hole spins along the cosmic history, due to the combination of mergers and accretion events. I describe recent simulations of accreting black holes in merger remnants, and discuss the implication for the spins of black holes in quasars.