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

TL;DR

This study uses MeerKAT radio observations to investigate HI absorption in 17 intermediate-redshift radio AGNs, revealing detection rates linked to source age, UV luminosity, and gas geometry, with implications for understanding AGN environments.

Contribution

First detailed MeerKAT survey of HI absorption in intermediate-redshift radio AGNs, connecting absorption features to AGN properties and gas conditions.

Findings

Detected HI absorption in 3 out of 17 sources.

Associated HI absorption more common in young, low UV luminosity sources.

HI absorption shows diverse origins, including circumnuclear discs and jet-driven outflows.

Abstract

We present MeerKAT observations searching for HI absorption in a sample of 17 powerful ($L_{\rm 1.4GHz}> 10^{27}$ 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 HI to the properties of the AGN and its host galaxy. The sample consists mostly of core-dominated sources and quasars. Half of the targets have a UV luminosity $L_{\rm UV} = 10^{23}$ W Hz$^{-1}$, 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 ($\tau_{\rm peak} < 0.005$) resulting in three new HI absorption detections. Two are associated HI absorptions, giving a detection rate of such systems of $13\%\pm 7\%$. Both are found in young radio sources (PKS 1151-34 and PKS 1306-09), confirming the trend that this type of sources are more often detected in HI compared to more evolved ones. The UV luminosity of both these sources is below $10^{23}$ W Hz$^{-1}$. Surprisingly, one of the detections (PKS 1151-34) is hosted by a quasar, suggesting that the radio lobes are still embedded in the circumnuclear disc. In the second source (PKS 1306-09), the HI is highly blueshifted and likely part of a jet-driven outflow. 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. 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 HI absorption.