2024A&A...687A..50D

We report a new, rare detection of H I 21 cm absorption associated with a quasar (only six quasars are known at 1 < z < 2) toward J2339-5523 at z_em = 1.3531, discovered through the MeerKAT Absorption Line Survey (MALS). The absorption profile is broad (∼400 km s^–1 ), and the peak is redshifted by ∼200 km s^–1 from z_em. Interestingly, optical/far-UV spectra of the quasar from the Magellan-MIKE/HST-COS spectrographs do not show any absorption features associated with the 21 cm absorption, despite the coincident presence of the optical quasar and the radio core inferred from a flat-spectrum component with a flux density of ∼65 mJy at high frequencies (> 5 GHz). The simplest explanation would be that no large H I column (N(H I) > 10¹⁷ 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_ - ΔV_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.