  Many models that seek to explain the origin of the unresolved X-ray
background predict that Compton-thick Active Galactic Nuclei (AGNs) are
ubiquitious at high redshift. However, few distant Compton-thick AGNs have been
reliably identified to date. Here we present Spitzer-IRS spectroscopy and
3.6-70um photometry of a z=2.2 optically identified AGN (HDF-oMD49) that is
formally undetected in the 2Ms Chandra Deep Field-North (CDF-N) survey. The
Spitzer-IRS spectrum and spectral energy distribution of this object is AGN
dominated, and a comparison of the energetics at X-ray wavelengths to those
derived from mid-infrared (mid-IR) and optical spectroscopy shows that the AGN
is intrinsically luminous (L_X~3x10^44 erg/s) but heavily absorbed by
Compton-thick material (N_H>>10^24 cm^{-2}); i.e., this object is a
Compton-thick quasar. Adopting the same approach that we applied to HDF-oMD49,
we found a further six objects at z~2-2.5 in the literature that are also X-ray
weak/undetected but have evidence for AGN activity from optical and/or mid-IR
spectroscopy, and show that all of these sources are also Compton-thick quasars
with L_X>10^44 erg/s. On the basis of the definition of Daddi etal. (2007),
these Compton-thick quasars would be classified as mid-IR excess galaxies, and
our study provides the first spectroscopic confirmation of Compton-thick AGN
activity in a subsample of these z~2 mid-IR bright galaxies. Using the four
objects that lie in the CDF-N field, we estimate the space-density of
Compton-thick quasars [Phi~(0.7-2.5)x10^-5 Mpc^-3 for L_X>10^44 erg/s objects
at z~2-2.5] and show that Compton-thick accretion is as ubiquitious as
unobscured accretion in the distant Universe.
