Giant Metrewave Radio Telescope detections of two high-opacity HI 21cm
  absorbers at $z \approx 1.2$
  We report the discovery of two remarkable high-opacity HI 21cm absorbers
against low-luminosity active galactic nuclei (AGNs), at $z = 1.2166$ towards
J0229+0044 and at $z=1.1630$ towards J0229+0053. The absorbers were detected in
an unbiased Giant Metrewave Radio Telescope survey for HI 21cm absorption
against radio sources in the DEEP2 survey fields, covering $z \approx
0.73-1.53$, and including sources without known redshifts. The
velocity-integrated HI 21cm optical depths are $(74.2 \pm 7.8)$ km s$^{-1}$
(J0229+0044) and $(78.41 \pm 0.81)$ km s$^{-1}$ (J0229+0053), higher than that
of any known redshifted HI 21cm absorber at $z > 0.12$, and implying high H{\sc
i} column densities, $> 10^{22}$ cm$^{-2}$. The emission redshift of J0229+0044
is consistent with the HI 21cm absorption redshift, while the strength and
velocity spread of the absorption against J0229+0053 suggest that it too arises
from gas in the AGN environment: both absorbers are thus likely to be
"associated" systems. The two AGNs have low rest-frame 1.4 GHz radio and 1215
Angstrom ultraviolet luminosities ($\lesssim 10^{26.1}$ W Hz$^{-1}$ and
$\lesssim 10^{21.7}$ W Hz$^{-1}$, respectively), both significantly lower than
the typical luminosities of AGNs against which HI 21cm searches have hitherto
been carried out at $z \gtrsim 1$. The paucity of HI 21cm absorbers at $z
\gtrsim 1$ may be due to a luminosity bias in high-$z$ AGN samples that have
been searched for HI 21cm absorption, where the high AGN ultraviolet luminosity
affects physical conditions in its environment, ionizing the neutral hydrogen.
