The XMM-Newton Wide field survey in the COSMOS field: redshift evolution
  of AGN bias and subdominant role of mergers in triggering moderate luminosity
  AGN at redshift up to 2.2
  We present a study of the redshift evolution of the projected correlation
function of 593 X-ray selected AGN with I_AB<23 and spectroscopic redshifts
z<4, extracted from the 0.5-2 keV X-ray mosaic of the 2.13 deg^2 XMM-COSMOS
survey. We introduce a method to estimate the average bias of the AGN sample
and the mass of AGN hosting halos, solving the sample variance using the halo
model and taking into account the growth of the structure over time. We find
evidence of a redshift evolution of the bias factor for the total population of
XMM-COSMOS AGN from b(z=0.92)=2.30 +/- 0.11 to b(z=1.94)=4.37 +/- 0.27 with an
average mass of the hosting DM halos logM [h^-1 M_sun] ~ 13.12 +/- 0.12 that
remains constant at all z < 2. Splitting our sample into broad optical lines
AGN (BL), AGN without broad optical lines (NL) and X-ray unobscured and
obscured AGN, we observe an increase of the bias with redshift in the range
z=0.7-2.25 and z=0.6-1.5 which corresponds to a constant halo mass logM [h^-1
M_sun] ~ 13.28 +/- 0.07 and logM [h^-1 M_sun] ~ 13.00 +/- 0.06 for BL /X-ray
unobscured AGN and NL/X-ray obscured AGN, respectively. The theoretical models
which assume a quasar phase triggered by major mergers can not reproduce the
high bias factors and DM halo masses found for X-ray selected BL AGN with L_BOL
~ 2e45 erg s^-1. Our work extends up to z ~ 2.2 the z <= 1 statement that, for
moderate luminosity X-ray selected BL AGN, the contribution from major mergers
is outnumbered by other processes, possibly secular such as tidal disruptions
or disk instabilities.
