What triggers black-hole growth? Insights from star formation rates
  We present a new semi-analytic model for the common growth of black holes
(BHs) and galaxies within a hierarchical Universe. The model is tuned to match
the mass function of BHs at z=0 and the luminosity functions of active galactic
nuclei (AGNs) at z<4. We use a new observational constraint, which relates the
luminosity of AGNs to the star-formation rate (SFR) of their host galaxies. We
show that this new constraint is important in various aspects: a) it indicates
that BH accretion events are episodic; b) it favours a scenario in which BH
accretion is triggered by merger events of all mass ratios; c) it constrains
the duration of both merger-induced star-bursts and BH accretion events. The
model reproduces the observations once we assume that only 4 per cent of the
merger events trigger BH accretion; BHs accretion is not related to secular
evolution; and only a few per cent of the mass made in bursts goes into the BH.
We find that AGNs with low or intermediate luminosity are mostly being
triggered by minor merger events, in broad agreement with observations. Our
model matches various observed properties of galaxies, such as the stellar mass
function at z<4 and the clustering of galaxies at redshift zero. This allows us
to use galaxies as a reliable backbone for BH growth, with reasonable estimates
for the frequency of merger events. Other modes of BH accretion, such as
disk-instability events, were not considered here, and should be further
examined in the future.
