  Using X-ray stacking analyses we estimate the average amounts of supermassive
black hole (SMBH) growth taking place in star-forming galaxies (SFGs) at z~1
and z~2 as a function of galaxy stellar mass (M*). We find the average SMBH
growth rate follows remarkably similar trends with M* and redshift as the
average star-formation rates (SFRs) of their host galaxies (i.e., (i.e.,
dM_BH/dt ~ M*^(0.86+/-0.39) for the z~1 sample and dM_BH/dt ~ M*^(1.05+/-0.36)
for the z~2 sample). It follows that the ratio of SMBH growth rate to SFR is
(a) flat with respect to M* (b) not evolving with redshift and (c) close to the
ratio required to maintain/establish a SMBH to M* ratio of ~10^{-3} as also
inferred from today's M_BH-M_bulge relationship. We interpret this as evidence
that SMBHs have, on average, grown in-step with their host galaxies since at
least z~2, irrespective of host galaxy mass and AGN triggering mechanism. As
such, we suggest that the same secular processes that drive the bulk of star
formation are also responsible for the majority of SMBH growth. From this, we
speculate that it is the availability of gas reservoirs that regulate both
cosmological SMBH growth and star formation.
