Giant Metrewave Radio Telescope Detections of Two High-opacity Hi 21 cm Absorbers at z 1.2

• Published in: Astrophysical journal. Letters Vol. 900; no. 2; p. L30
• Main Authors: Chowdhury, Aditya, Kanekar, Nissim, Chengalur, Jayaram N.
• Format: Journal Article
• Language: English
• Published: Austin The American Astronomical Society 01.09.2020; IOP Publishing
• Subjects: Absorbers; Absorption; Active galactic nuclei; Hydrogen; Luminosity; Neutral hydrogen clouds; Opacity; Polls & surveys; Radio spectroscopy; Radio telescopes; Red shift; Velocity
• ISSN: 2041-8205; 2041-8213
• DOI: 10.3847/2041-8213/abb13d

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 ultraviolet luminosities ( 1026.1 W Hz−1 and 1021.7 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.