2020/08/26 by Aditya Chowdhury, Chowdhury, Aditya, Nissim Kanekar +4 · 1 citation
Physics and Astronomy · #Astrophysics and Cosmic Phenomena #Astrophysics of Galaxies (astro-ph.GA) #Cosmology and Nongalactic Astrophysics (astro-ph.CO) #FOS: Physical sciences #Galaxies: Formation, Evolution, Phenomena #Radio Astronomy Observations and Technology #astro-ph.CO #astro-ph.GA
paper · pdf · doi:10.48550/arxiv.2008.11403
8 pages, 2 figures; Accepted for publication in ApJ Letters
arxiv created 2020/08/26 · openalex publication_date 2020/08/26 · arxiv updated 2020/08/27 · openalex created_date 2020/09/01 · openalex updated_date 2026/07/28
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 ≈ 0.73-1.53, and including sources without known redshifts. The velocity-integrated HI 21cm 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 21cm absorber at z > 0.12, and implying high H\sc i column densities, > 1022 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 1026.1 W Hz-1 and \lesssim 1021.7 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.