  We study the link between supermassive black hole growth and the stellar mass
assembly of their host galaxies in the state-of-the-art Romulus suite of
simulations. The cosmological simulations Romulus25 and RomulusC employ
innovative recipes for the seeding, accretion, and dynamics of black holes in
the field and cluster environments respectively. We find that the black hole
accretion rate traces the star formation rate among star-forming galaxies. This
result holds for stellar masses between 10^8 and 10^12 solar masses, with a
very weak dependence on host halo mass or redshift. The inferred relation
between accretion rate and star formation rate does not appear to depend on
environment, as no difference is seen in the cluster/proto-cluster volume
compared to the field. A model including the star formation rate, the black
hole-to-stellar mass ratio, and the cold gas fraction can explain about 70 per
cent of all variations in the black hole accretion rate among star forming
galaxies. Finally, bearing in mind the limited volume and resolution of these
cosmological simulations, we find no evidence for a connection between black
hole growth and galaxy mergers, on any timescale and at any redshift. Black
holes and their galaxies assemble in tandem in these simulations, regardless of
the larger-scale intergalactic environment, suggesting that black hole growth
simply follows star formation on galactic scales.

Astrophysics of Galaxies