  We investigate the NV absorption signal along the line of sight of background
quasars, in order to test the robustness of the use of this ion as criterion to
select intrinsic (i.e. physically related to the quasar host galaxy) narrow
absorption lines (NALs). We build composite spectra from a sample of $\sim$
1000 CIV absorbers, covering the redshift range 2.55 < z < 4.73, identified in
100 individual sight lines from the XQ-100 Legacy Survey. We detect a
statistical significant NV absorption signal only within 5000 km s$^{-1}$ of
the systemic redshift, z$\rm_{em}$. This absorption trough is $\sim$ 15$\sigma$
when only CIV systems with N(CIV) > 10$^{14}$ cm$^{-2}$ are included in the
composite spectrum. This result confirms that NV offers an excellent
statistical tool to identify intrinsic systems. We exploit the stacks of 11
different ions to show that the gas in proximity to a quasar exhibits a
considerably different ionization state with respect to gas in the transverse
direction and intervening gas at large velocity separations from the continuum
source. Indeed, we find a dearth of cool gas, as traced by low-ionization
species and in particular by MgII, in the proximity of the quasar. We compare
our findings with the predictions given by a range of Cloudy ionization models
and find that they can be naturally explained by ionization effects of the
quasar.

Astrophysics of Galaxies