Triggering Active Galactic Nuclei in Hierarchical Galaxy Formation: Disk
  instability vs. Interactions
  Using a semi analytic model for galaxy formation we investigate the effects
of Black Hole accretion triggered by disk instabilities (DI) in isolated
galaxies on the evolution of AGN. Specifically, we took on, developed and
expanded the Hopkins & Quataert (2011) model for the mass inflow following disk
perturbations, and compare the corresponding evolution of the AGN population
with that arising in a scenario where galaxy interactions trigger AGN (IT
mode). We extended and developed the DI model by including different disk
surface density profiles, to study the maximal contribution of DI to the
evolution of the AGN population. We obtained the following results: i) for
luminosities corresponding to $M_{1450}\gtrsim -26$ the DI mode can provide the
BH accretion needed to match the observed AGN luminosity functions up to $z
\approx 4.5$; in such a luminosity range and redshift, it can compete with the
IT scenario as the main driver of cosmological evolution of AGN; ii) The DI
scenario cannot provide the observed abundance of high-luminosity QSO with
$M_{1450}\lesssim -26$ AGN, as well as the abundance of high-redhshift $z
\approx 4.5$ QSOs with $M_{1450}\lesssim -24$, while the IT scenario provides
an acceptable match up to $z \approx 6$, as found in our earliest works; iii)
The dispersion of the distributions of Eddington ratio for low- and
intermediate-luminosity AGN (bolometric $L_{AGN}$ = $10^{43}$ - $10^{45}$
erg/s) is predicted to be much smaller in the DI scenario compared to the IT
mode; iv) The above conclusions are robust with respect to the explored
variants of the Hopkins & Quataert (2011) model. We discuss the physical origin
of our findings, and how it is possible to pin down the dominant fueling
mechanism in the low-intermediate luminosity range $M_{1450}\gtrsim -26$ where
both the DI and the IT modes are viable candidates as drivers for the AGN
evolution.
