In radio galaxies, an understanding of the gaseous
components in their environments as well as in the
nuclear regions could provide insights towards
understanding the triggering of radio activity,
understanding fueling processes, probing jet-cloud
interactions and also constraining orientation dependent unification schemes.
Detection of HI via 21-cm absorption lines towards
compact radio sources (GPS, CSS and compact cores of
the larger radio sources) has been an important tool
to study this cold neutral component of the
circumnuclear gas in the host galaxies of AGN.
This gas could be either part of the interstellar
medium (ISM) of the host galaxy, or be a constituent
of the circumnuclear tori or disk in the central
region of the host. I present the results of HI 21-cm
absorption line studies towards the central
regions of radio galaxies. These observations were
made with the Giant Metrewave Radio
Telescope(GMRT) to detect HI gas via absorption, study the kinematics and distribution of this cold atomic
gas in their central regions. We detected new HI
absorption towards seven radio sources in our studies
done for a sample of low luminosity nearby compact
CSS & GPS radio sources. Within the uncertainties, the
detection rates and column densities are similar to
the more luminous objects, with the GPS objects exhibiting a higher detection rate than for the CSS
objects. In HI absorption studies towards the
cores of larger radio sources, considering
observations of similar sensitivity, we find
that the detection rate of HI absorption towards the
cores of larger (>15 kpc) sources is only ∼15 per cent, compared with ∼57 per cent for CSS and GPS
objects, suggesting an evolution in the gaseous
content of the host galaxies as the radio sources age
and grow in size. I will also summarize the results of
our HI absorption observations towards the central
regions of rejuvenated radio sources.