Giant Metrewave Radio Telescope detections of two high-opacity HI 21cm absorbers at z ≈ 1.2

Abstract

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 (lesssim1026.1 W Hz^-1 and lesssim1021.7 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 gsim 1. The paucity of Hi 21 cm absorbers at z gsim 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.