Blandford-Znajek jets in galaxy formation simulations: method and
  implementation
  Jets launched by active galactic nuclei (AGN) are believed to play a
significant role in shaping the properties of galaxies and provide an
energetically viable mechanism through which galaxies can become quenched. Here
we present a novel AGN feedback model, which we have incorporated into the
AREPO code, that evolves the black hole mass and spin as the accretion flow
proceeds through a thin $\alpha$-disc which we self-consistently couple to a
Blandford-Znajek jet. We apply our model to the central region of a typical
radio-loud Seyfert galaxy embedded in a hot circumgalactic medium (CGM). We
find that jets launched into high pressure environments thermalise efficiently
due to the formation of recollimation shocks and the vigorous instabilities
that these shocks excite increase the efficiency of the mixing of CGM and jet
material. The beams of more overpressured jets, however, are not as readily
disrupted by instabilities so the majority of the momentum flux at the jet base
is retained out to the head, where the jet terminates in a reverse shock. All
jets entrain a significant amount of cold circumnuclear disc material which,
while energetically insignificant, dominates the lobe mass together with the
hot, entrained CGM material. The jet power evolves significantly due to
effective self-regulation by the black hole, fed by secularly-driven,
intermittent mass flows. The direction of jets launched directly into the
circumnuclear disc changes considerably due to effective Bardeen-Petterson
torquing. Interestingly, these jets obliterate the innermost regions of the
disc and drive large-scale, multi-phase, turbulent, bipolar outflows.
