The observed local correlation between black-hole masses and galaxy bulge
masses suggests that nuclear and star formation activity should have been
related at some point in the past. Investigating the incidence of AGN among
high-redshift galaxies is then crucial to understand whether this idea is
actually correct. X-ray data are excellent to reveal the AGN presence, but
some highly obscured AGN, or low-luminosity AGN, can be undetected even in
typically deep X-ray maps. Searching for a hot-dust component at infarred
(IR) wavelengths offers a powerful alternative to investigate the AGN
incidence in large galaxy surveys. In this talk I will present a new
diagnostic to unveil the presence of AGN in IR galaxies at z>1, down to
flux densities at which the extragalactic IR background is mostly resolved.
This diagnostic is based on the analysis of the optical/IR
spectral energy distributions of 174 Spitzer galaxies selected with
S(24microns)> 80 microJy, all with secure spectroscopic redshifts zspec>1,
in the Chandra Deep Field South. I will also show an independent analysis
of the AGN presence in these sources performed with a combination of
ultra-deep X-ray images, host galaxy morphologies, and modelled dust
temperatures. As a conclusion, I will argue that my results indicate that
between ~30% and ~50% of IR sources contain an AGN at z=1.0-1.5.