  Using an advanced semi analytic model (SAM) for galaxy formation, we have
investigated the statistical effects of assuming two different mechanisms for
triggering AGN activity on the properties of AGN host galaxies. We have
considered a first accretion mode where AGN activity is triggered by disk
instabilities (DI) in isolated galaxies, and a second feeding mode where such
an activity is triggered by galaxy mergers and fly-by events (interactions,
IT). We obtained the following results:i) for hosts with $M_* \lesssim 10^{11}
M_{\bigodot}$, both DI and IT modes are able to account for the observed AGN
hosts stellar mass function; for more massive hosts, the DI scenario predicts a
lower space density than the IT model, lying below the observational estimates
for z>0.8.ii) The analysis of the color-magnitude diagram (CMD) of AGN hosts
for redshift z < 1.5 can provide a good observational test to effectively
discriminate between the DI and IT mode, since DIs are expected to yield AGN
host galaxy colors skewed towards bluer colors, while in the IT scenario the
majority of hosts are expected to reside in the red sequence.iii) While both IT
and DI scenarios can account for AGN triggered in main sequence or starburst
galaxies, DIs fail in triggering AGN activity in passive galaxies.iv) The two
modes are characterized by a different duration of the AGN phase, with DIs
lasting even on time scales $\sim $ Gyr, much longer with respect to the IT
scenario.v) The scatter of the $SFR-L_{bol}$ relation could represent another
crucial diagnostics to discriminate between the two triggering modes, since the
DI scenario predicts an appreciably lower scatter of the relation than the IT
scenario. vi) Disk instabilities are not able to account for the observed
fraction of AGN in groups for z < 1 and clusters for z < 0.7, while the IT
scenario provides a good match to observational data.
