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

TL;DR

This paper reports the discovery of two high-opacity HI 21cm absorbers at around redshift 1.2, detected in an unbiased GMRT survey, revealing high neutral hydrogen column densities in low-luminosity AGNs and suggesting a luminosity bias in previous high-redshift HI absorption studies.

Contribution

First detection of high-opacity HI 21cm absorbers in low-luminosity AGNs at z~1.2, highlighting potential biases in earlier surveys targeting high-luminosity AGNs.

Findings

Both absorbers have optical depths higher than any known at z > 0.12.

The absorbers indicate high H I column densities (>10^{22} cm^{-2}).

The low luminosity of the AGNs suggests previous non-detections may be due to luminosity bias.

Abstract

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.