2021A&A...654A..94M

We present an HI 21 cm absorption study of a sample of 26 radio-loud active galactic nuclei (AGN) at 0.25 < z < 0.4 carried out with the Karl G. Jansky Very Large Array. Our aim was to study the rate of incidence of HI in different classes of radio AGN, the morphology and kinematics of the detected HI, and the nature of the interaction between the HI and the radio source at these redshifts. Our sample consists of 14 sources with sizes of up to tens of kpc and 12 compact sources (< a few kpc) in the radio-power range 10^25.7 W Hz^–1-10^26.5 W Hz^–1. We detect HI in five sources, corresponding to a detection rate of ∼19%. Within the error bars, this agrees with the detection rate found at lower redshifts. We find that the rest-frame UV luminosities of most of the sources in the sample, including all the detections, are below the proposed threshold above which the HI is supposed to have been ionised. An analysis of the optical emission-line spectra of the sources shows that despite their high radio powers, about one-third of the sample, including two detections, are low-ionisation sources. The radio continuum emission from the sources detected in HI is unresolved at ∼5 to 10 kpc scales in our observations, but shows extended structure on parsec scales. We analysed the HI 21 cm absorption spectra of the detections to understand the morphology and kinematics of HI. The absorption profiles are mostly complex with widths between the nulls ranging from ∼60 km s^–1 to 700 km s^–1. These detections also exhibit remarkably high HI column densities in the range ∼10²¹ cm^–2-10²² cm^–2 for T_spin = 100 K and unit covering factor. Our modelling of the HI 21 cm absorption profiles suggests that in two sources the gas appears to be disturbed, and in three cases, including one with disturbed HI, the majority of the absorption is consistent with it arising from an HI disc. Despite the high radio power of our sources, we do not detect fast outflows. However, the optical emission lines in these detections show the presence of significantly disturbed gas in the nuclear regions in the form of very wide and highly blueshifted emission-line components. Since some of our detections are also low-ionisation AGN, it is quite possible that this disturbance is caused by the radio jets. Overall, our findings point towards a continuation of the low-z trends in the HI detection rate and the incidence of HI in radio AGN up to z∼0.4.