Running Late: Testing Delayed Supermassive Black Hole Growth Models
  Against the Quasar Luminosity Function
  Observations of massive galaxies at low redshift have revealed approximately
linear scaling relations between the mass of a supermassive black hole (SMBH)
and properties of its host galaxy. How these scaling relations evolve with
redshift and whether they extend to lower-mass galaxies however remain open
questions. Recent galaxy formation simulations predict a delayed, or
"two-phase", growth of SMBHs: slow, highly intermittent BH growth due to
repeated gas ejection by stellar feedback in low-mass galaxies, followed by
more sustained gas accretion that eventually brings BHs onto the local scaling
relations. The predicted two-phase growth implies a steep increase, or "kink",
in BH-galaxy scaling relations at a stellar mass $M_{*}\sim5\times10^{10}
M_{\odot}$. We develop a parametric, semi-analytic model to compare different
SMBH growth models against observations of the quasar luminosity function (QLF)
at $z\sim0.5-4$. We compare models in which the relation between SMBH mass and
galaxy mass is purely linear versus two-phase models. The models are anchored
to the observed galaxy stellar mass function, and the BH mass functions at
different redshifts are consistently connected by the accretion rates
contributing to the QLF. The best fits suggest that two-phase evolution is
significantly preferred by the QLF data over a purely linear scaling relation.
Moreover, when the model parameters are left free, the two-phase model fits
imply a transition mass consistent with that predicted by simulations. Our
analysis motivates further observational tests, including measurements of BH
masses and AGN activity at the low-mass end, which could more directly test
two-phase SMBH growth.
