Black hole spin and radio loudness in a LCDM universe

TL;DR

This study combines cosmological simulations and semi-analytic models to explore how supermassive black hole spins evolve and relate to galaxy properties and radio loudness, highlighting the dominant role of accretion history.

Contribution

It introduces a model linking black hole spin development to accretion processes and galaxy features, emphasizing the impact of accretion history over disc alignment on spin evolution.

Findings

More massive black holes have higher spins and reside in elliptical galaxies.

Radio-loud AGN fractions increase with black hole and stellar mass when considering spin limits.

Gas accretion and disc instabilities are primary mechanisms for black hole spin-up.

Abstract

We use a combination of a cosmological N-body simulation of the concordance Lambda cold dark matter (LCDM) paradigm and a semi-analytic model of galaxy formation to investigate the spin development of central supermassive black holes (BHs) and its relation to the BH host galaxy properties. In order to compute BH spins, we use the alpha-model of Shakura & Sunyaev and consider the King et al. warped disc alignment criterion. The orientation of the accretion disc is inferred from the angular momentum of the source of accreted material, which bears a close relationship to the large-scale structure in the simulation. We find that the final BH spin depends almost exclusively on the accretion history and only weakly on the warped disc alignment. The main mechanisms of BH spin-up are found to be gas cooling processes and disc instabilities, a result that is only partially compatible with Monte-Carlo models where the main spin-up mechanisms are major mergers and disc instabilities; the latter results are reproduced when implementing randomly oriented accretion discs in our model. Regarding the BH population, we find that more massive BHs, which are hosted by massive ellipticals, have higher spin values than less-massive BHs, hosted by spiral galaxies. We analyse whether gas accretion rates and BH spins can be used as tracers of the radio loudness of active galactic nuclei (AGN). We find that the current observational indications of an increasing trend of radio-loud AGN fractions with stellar and BH mass can be easily obtained when placing lower limits on the BH spin, with a minimum influence from limits on the accretion rates; a model with random accretion disc orientations is unable to reproduce this trend. (ABRIDGED)