Giant Metrewave Radio Telescope detection of associated H i 21-cm absorption atz = 1.2230 towards TXS 1954+513

Abstract

We have used the 610-MHz receivers of the Giant Metrewave Radio Telescope (GMRT) to detect associated H I 21-cm absorption from the z = 1.2230 blazar TXS 1954+513. The GMRT H I 21-cm absorption is likely to arise against either the milliarcsecond-scale core or the one-sided milliarcsecond-scale radio jet, and is blueshifted by ≈328 km s^-11 from the blazar redshift. This is consistent with a scenario in which the H I cloud giving rise to the absorption is being driven outwards by the radio jet. The integrated H I 21-cm optical depth is (0.716 ± 0.037) km s^-1, implying a high H I column density, N_HI=(1.305±0.067)×(T_s/100K)×10²⁰ cm^-2, for an assumed H I spin temperature of 100 K. We use Nickel Telescope photometry of TXS 1954+513 to infer a high rest-frame 1216 Å luminosity of (4.1 ± 1.2) × 10²³ W Hz^-1. The z = 1.2230 absorber towards TXS 1954+513 is only the fifth case of a detection of associated H I 21^-cm 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 (UV) luminosity ≫10²³ W Hz^-1 , demonstrating that neutral hydrogen can survive in AGN environments in the presence of high UV luminosities.