HI absorption towards low luminosity radio-loud AGNs of different accretion modes and WISE colours

Abstract

H{\sc i} absorption studies of active galaxies enable us to probe their circumnuclear regions and the general interstellar medium, and study the supply of gas which may trigger the nuclear activity. In this paper, we investigate the detection rate of H{\sc i} absorption on the nature of radio galaxies based on their emission-line spectra, nature of the host galaxies based on the \textit{WISE} colours and their radio structure, which may help understand the different accretion modes. We find significant difference in distributions of W2$-$W3 colour for sources with H{\sc i} absorption detections and non-detections. We report a high detection rate of H{\sc i} absorption in the galaxies with \textit{WISE} infrared colours W2$-$W3 $>$ 2, which is typical of gas-rich systems, along with a compact radio structure. The H{\sc i} detection rate for low-excitation radio galaxies (LERGs) with W2$-$W3 $>$ 2 and compact radio structure is high (70.6$\pm$20.4 \%). In HERGs, compact radio structure in the nuclear or circumnuclear region could give rise to absorption by gas in the dusty torus in addition to gas in the interstellar medium. However, higher specific star formation rate (sSFR) for the LERGs with W2$-$W3 $>$ 2 suggests that H{\sc i} 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 to those with compact structure. Extended radio sources and those with W2$-$W3 $<$ 2 have low H{\sc i} detection rates.