Black hole -- galaxy scaling relations in FIRE: the importance of black
  hole location and mergers
  The concurrent growth of supermassive black holes (SMBHs) and their host
galaxies remains to be fully explored, especially at high redshift. While often
understood as a consequence of self-regulation via AGN feedback, it can also be
explained by alternative SMBH accretion models. Here, we expand on previous
work by studying the growth of SMBHs with the help of a large suite of
cosmological zoom-in simulations (MassiveFIRE) that are part of the Feedback in
Realistic Environments (FIRE) project. The growth of SMBHs is modelled in
post-processing with different black hole accretion models, placements, and
merger treatments, and validated by comparing to on-the-fly calculations.
Scaling relations predicted by the gravitational torque driven accretion (GTDA)
model agree with observations at low redshift without the need for AGN
feedback, in contrast to models in which the accretion rate depends strongly on
SMBH mass. At high redshift, we find deviations from the local scaling
relations in line with previous theoretical results. In particular, SMBHs are
under-massive, presumably due to stellar feedback, but start to grow
efficiently once their host galaxies reach $M_* \sim 10^{10} M_{\odot}$. We
analyse and explain these findings in the context of a simple analytic model.
Finally, we show that the predicted scaling relations depend sensitively on the
SMBH location and the efficiency of SMBH merging, particularly in low-mass
systems. These findings highlight the relevance of understanding the evolution
of SMBH-galaxy scaling relations to predict the rate of gravitational wave
signals from SMBH mergers across cosmic history.
