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

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 ($τ_{\rm peak} < 0.005$) 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.

💡 Research Summary

This paper presents a systematic search for neutral hydrogen (HI) 21‑cm absorption in a well‑characterised sample of 17 powerful radio AGN selected from the southern 2‑Jy catalogue, spanning the intermediate redshift range 0.25 < z < 0.7. The sources were chosen to have a core flux density >30 mJy at 1.4 GHz, resulting in a bias toward core‑dominated quasars and a small number of radio galaxies, with four objects classified as compact steep‑spectrum (CSS) sources – the archetypal young radio AGN.

Observations were carried out with the MeerKAT array in the L‑band (≈ 900 MHz), delivering a bandwidth of ~11 MHz split into 2048 channels (≈ 10 km s⁻¹ resolution). After careful RFI excision and band‑pass calibration, the authors obtained continuum images with beam sizes of ~9 × 8 arcsec and line sensitivities of 0.3–1.0 mJy beam⁻¹, corresponding to peak optical depths τₚₑₐₖ < 0.005 for the brightest sources – a factor of two deeper than most previous studies at similar redshifts.

Out of the 15 spectra that were essentially free of RFI, three new HI absorption systems were identified. Two are associated with the host galaxies of the radio AGN (detection rate 13 % ± 7 %). Both associated absorbers are found in young radio sources (PKS 1151‑34 and PKS 1306‑09). Their ultraviolet luminosities lie below the often‑cited ionisation threshold of L_UV = 10²³ W Hz⁻¹, supporting the hypothesis that strong UV fields can ionise the neutral gas and suppress absorption. PKS 1151‑34 is a quasar, yet the detection implies that its radio lobes are still embedded within a circumnuclear disc, a configuration more typical of radio galaxies. PKS 1306‑09 shows a highly blueshifted absorption component (≈ ‑800 km s⁻¹), consistent with a jet‑driven outflow that has already been seen in warm ionised gas, providing a rare example of a multi‑phase outflow in a young AGN.

The third detection is a “local intervening” absorber: a foreground galaxy in the environment of PKS 0405‑12 lies in front of the southern radio lobe, at a projected distance of ~100 kpc. This illustrates the importance of high‑resolution imaging and ancillary data to distinguish true associated absorption from unrelated foreground systems, especially in blind surveys.

The authors discuss three principal factors governing the detection of HI absorption at these redshifts: (1) the evolutionary stage of the radio source – compact, young sources have a higher covering factor of the background continuum by the host‑galaxy gas; (2) the strength of the UV and radio radiation fields – high L_UV or intense radio illumination raises the spin temperature and reduces τ for a given column density; (3) the geometry of the neutral gas – a rotating disc or ring intersecting the line of sight yields narrow, deep lines, whereas off‑axis clouds or outflows produce broader, shifted features. Their findings align with low‑z studies, indicating that the same physical mechanisms operate out to z ≈ 0.7.

Methodologically, the work showcases MeerKAT’s capability to reach τ ≈ 0.005 in reasonable integration times, opening a window on a population of shallow absorbers that were previously missed. The authors anticipate that upcoming blind HI absorption surveys such as FLASH, combined with MeerKAT’s sensitivity, will dramatically increase the sample size at z > 0.2, allowing robust statistical tests of the UV‑luminosity threshold, the prevalence of jet‑driven outflows, and the role of host‑galaxy morphology.

In conclusion, the study confirms that at intermediate redshifts the detection of associated HI absorption is primarily dictated by the youth of the radio source, modest UV luminosity, and favorable gas geometry, mirroring trends observed locally. The successful detection of shallow absorbers demonstrates MeerKAT’s suitability for high‑redshift HI absorption work and sets the stage for future large‑scale investigations of the cold neutral medium in AGN host galaxies.