MALS discovery of a rare H I 21 cm absorber at z ∼ 1.35: Origin of the absorbing gas in powerful active galactic nuclei

We report a new, rare detection of H I 21 cm absorption associated with a quasar (only six quasars are known at 1 10 17 cm −2 ) is present toward the radio core and the optical active galactic nucleus. Based on the joint optical and radio analysis of a heterogeneous sample of 16 quasars ( z median = 0.7) and 19 radio galaxies ( z median = 0.4) with H I 21 cm absorption detection and matched in 1.4 GHz luminosity ( L 1.4 GHz ), a consistent picture emerges according to which quasars primarily trace the gas in the inner circumnuclear disk and cocoon created by the interaction of the jet with interstellar medium. They (i.e., quasars) exhibit a L 1.4 GHz – Δ V null correlation and a frequent mismatch of the radio and optical spectral lines. The radio galaxies show no such correlation and likely trace the gas from the cocoon and the galaxy-wide interstellar medium outside the photoionization cone. The analysis presented here demonstrates the potential of radio spectroscopic observations to reveal the origin of the absorbing gas associated with active galactic nuclei that may be missed in optical observations.