A deep MeerKAT view of associated H I absorption in radio AGNs at intermediate redshift: Role of absorber geometry and conditions of the gas

¹ ASTRON, The Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo, The Netherlands
² Kapteyn Astronomical Institute, University of Groningen, Postbus 800, 9700 AV, Groningen, The Netherlands
³ Department of Physics and Astronomy, University of Sheffield, Sheffield S7 3RH, UK
⁴ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura, Casilla 19001, Santiago de Chile, Chile
⁵ Joint Institute for VLBI ERIC, Oude Hoogeveensedijk 4, 7991 PD, Dwingeloo, The Netherlands

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 22 December 2025
Accepted: 1 February 2026

Abstract

We present MeerKAT observations searching for H I absorption in a sample of 17 powerful (L_1.4 GHz > 10²⁷ W Hz⁻¹) radio sources at intermediate redshifts (0.25 < z < 0.7). The sample is well characterised at radio and optical wavelengths, allowing us to connect the presence (or absence) of H I to the properties of the active galactic nucleus (AGN) and its host galaxy. The sample consists mostly of core-dominated sources and quasars. Half of the targets have a UV luminosity below the limit of L_UV = 10²³ W Hz⁻¹, whereby at values above this limit, the gas would be expected to be ionised by this radiation. We obtained 15 spectra free (or almost free) of radio frequency interference, reaching extremely low optical depths (τ_peak < 0.005) resulting in three new H I absorption detections. Two are associated H I absorptions, giving a detection rate of such systems of 13%±7%. Both are found in a young radio source (PKS 1151–34 and PKS 1306–09), confirming the trend that this type of sources are more often detected in H I compared to more evolved ones. The UV luminosity of both these sources is below 10²³ W Hz⁻¹. Surprisingly, one of the detections (PKS 1151–34) is hosted by a quasar, suggesting that the radio lobes of this source are still embedded in the circumnuclear disc. In the second source (PKS 1306–09), the H I is highly blueshifted and likely part of the jet-driven outflow earlier observed in the warm ionised gas. This represents a new addition to the group of young radio AGNs, where multi-phased outflows have been observed as predicted by numerical simulations. A third detection is a ‘local intervening’ system, caused by a galaxy in the local environment of PKS 0405–12 and located in front of the southern radio lobe of this source, about 100 kpc in projection from this quasar. More such cases are expected to show up in large, blind surveys and our results show the need for high spatial resolution and good ancillary data to separate associated from intervening absorption. Overall, the results indicate a variety of plausible situations, which resemble what is seen at low redshifts. For the associated absorption, a combination of evolutionary status of the radio sources, physical conditions, and geometry of the gas structure determine the detection rate of H I absorption. The data also show the excellent capabilities of MeerKAT for obtaining very low optical depth detections, revealing the presence of an otherwise missed group of absorptions.