2020ApJ...900L..30C

We report the discovery of two remarkable high-opacity HI 21 cm absorbers against low-luminosity active galactic nuclei (AGNs), at z = 1.2166 toward J0229+0044 and at z = 1.1630 toward J0229+0053. The absorbers were detected in an unbiased Giant Metrewave Radio Telescope survey for HI 21 cm absorption against radio sources in the DEEP2 survey fields, covering z ≃ 0.73-1.53, and including sources without known redshifts. The velocity-integrated HI 21 cm optical depths are (74.2 ± 7.8) km s^–1 (J0229+0044) and (78.41 ± 0.81) km s^–1 (J0229+0053), higher than that of any known redshifted HI 21 cm absorber at z > 0.12, and implying high HI column densities, >10²² cm^–2. The emission redshift of J0229+0044 is consistent with the HI 21 cm 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 Å ultraviolet luminosities (<=10^26.1 W Hz^–1 and <=10^21.7 W Hz^–1, respectively), both significantly lower than the typical luminosities of AGNs against which HI 21 cm searches have hitherto been carried out at z >= 1. The paucity of HI 21 cm absorbers at z >= 1 may be due to a luminosity bias in high-z AGN samples that have been searched for HI 21 cm absorption, where the high AGN ultraviolet luminosity affects physical conditions in its environment, ionizing the neutral hydrogen.