Giant Metrewave Radio Telescope detection of associated HI 21cm absorption at $z = 1.2230$ towards TXS1954+513

TL;DR

This paper reports the first detection of associated HI 21cm absorption at z > 1 towards a luminous AGN, revealing neutral hydrogen can exist in high UV luminosity environments and suggesting jet-driven outflows.

Contribution

First detection of associated HI 21cm absorption at z > 1 in a high-luminosity AGN, expanding understanding of neutral hydrogen presence in active galactic nuclei environments.

Findings

Detected HI 21cm absorption at z=1.2230 towards TXS1954+513

Absorption likely arises against the radio core or jet, blueshifted by 328 km/s

High HI column density inferred, indicating abundant neutral hydrogen

Abstract

We have used the 610 MHz receivers of the Giant Metrewave Radio Telescope (GMRT) to detect associated HI 21cm absorption from the $z = 1.2230$ blazar TXS1954+513. The GMRT HI 21cm absorption is likely to arise against either the milli-arcsecond-scale core or the one-sided milli-arcsecond-scale radio jet, and is blueshifted by $\approx 328$ km s$^{-1}$ from the blazar redshift. This is consistent with a scenario in which the HI cloud giving rise to the absorption is being driven outward by the radio jet. The integrated HI 21cm optical depth is $(0.716 \pm 0.037)$ km s$^{-1}$, implying a high HI column density, $N_{\rm HI} = (1.305 \pm 0.067) \times ({\rm T_s/100\: K}) \times 10^{20}$ cm$^{-2}$, for an assumed HI spin temperature of 100 K. We use Nickel Telescope photometry of TXS1954+513 to infer a high rest-frame 1216 \AA\ luminosity of $(4.1 \pm 1.2) \times 10^{23}$ W Hz$^{-1}$. The $z = 1.2230$ absorber towards TXS1954+513 is only the fifth case of a detection of associated HI 21cm absorption at $z > 1$, and is also the first case of such a detection towards an active galactic nucleus (AGN) with a rest-frame ultraviolet luminosity $\gg 10^{23}$ W Hz$^{-1}$, demonstrating that neutral hydrogen can survive in AGN environments in the presence of high ultraviolet luminosities.