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Description
HI absorption studies of active galaxies enable us to probe their circumnuclear regions and the general interstellar medium and study the supply of gas that may trigger nuclear activity. In this article, we investigate the dependence of the detection rate of HI absorption on the nature of radio galaxies based on their emission-line spectra and on the nature of host galaxies based on WISE colours and their radio structure, which may help us understand the different accretion modes. We find significant differences in the distributions of W2-W3 colour for sources with HI absorption detections and non-detections. We report a high detection rate of HI absorption in those galaxies with WISE infrared colours W2-W3>2, typical of gas-rich systems, along with a compact radio structure. The HI detection rate for low-excitation radio galaxies (LERGs) with W2-W3>2 and compact radio structure is high (70.6+/-20.4 per cent). In high-excitation radio galaxies (HERGs), compact radio structure in the nuclear or circumnuclear region could give rise to absorption by gas in the dusty torus, in addition to gas in the interstellar medium. However, the higher specific star-formation rate (sSFR) for LERGs with W2-W3>2 suggests that HI absorption may be largely due to star-forming gas in their hosts. LERGs with extended radio structure tend to have significantly lower values of W2-W3 compared with those with compact structure. Extended radio sources and those with W2-W3<2 have low HI detection rates.
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