Black hole spin and radio loudness in a LCDM universe
  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)
