2020MNRAS.494.5161C

We present results of H I absorption experiment done using the Giant Metrewave Radio Telescope (GMRT) towards 27 low- and intermediate-luminosity (P_ 1.4 GHz_ ∼10²³-10²⁶ W Hz^–1) radio active galactic nuclei (AGNs), classified as either low excitation radio galaxies (LERGs) or high excitation radio galaxies (HERGs) and with WISE colour W2[4.6 µm]-W3[12 µm]> 2. We report H I absorption detection towards seven radio AGNs, six of which are new. Combined with other sources from literature classified as LERGs or HERGs, we confirm our earlier result that compact radio AGNs with WISE colour W2-W3 > 2 have higher detection rates compared to those with W2-W3 < 2. We find that H I absorption detection rate is higher for HERGs (37.0^+15.8_–11.5 per cent) compared to LERGs (22.0^+3.9_–3.4 per cent), mainly due to a larger fraction of HERGs being gas and dust rich with a younger stellar population compared to LERGs. However, for similar compact radio structures and host galaxies with WISE colours W2-W3 > 2, we don't find any significant difference in detection rates of two types of AGNs implying detection of H I gas may not necessarily mean high excitation mode AGN. We further analysed the kinematics towards these sources. We find that while LERGs show a wide range in the shift of centroid velocities (∼-479 to +356 km s^–1) relative to the optical systemic velocity, most of the HERGs have centroid velocity shift less than 200 km s^–1, possibly due to differences in jet-interstellar medium interaction.