Associated 21-cm absorption towards the cores of radio galaxies
  We present the results of Giant Metrewave Radio Telescope (GMRT) observations
to detect H{\sc i} in absorption towards the cores of a sample of radio
galaxies. From observations of a sample of 16 sources, we detect H{\sc i} in
absorption towards the core of only one source, the FR\,II radio galaxy 3C\,452
which has been reported earlier by Gupta & Saikia (2006a). In this paper we
present the results for the remaining sources which have been observed to a
similar optical depth as for a comparison sample of compact steep-spectrum
(CSS) and giga-hertz peaked spectrum (GPS) sources. We also compile available
information on H{\sc i} absorption towards the cores of extended radio sources
observed with angular resolutions of a few arcsec or better. The fraction of
extended sources with detection of H{\sc i} absorption towards their cores is
significantly smaller (7/47) than the fraction of H{\sc i} detection towards
CSS and GPS objects (28/49). For the cores of extended sources, there is no
evidence of a significant correlation between H{\sc i} column density towards
the cores and the largest linear size of the sources. The distribution of the
relative velocity of the principal absorbing component towards the cores of
extended sources is not significantly different from that of the CSS and GPS
objects. However, a few of the CSS and GPS objects have blue-shifted components
$\gapp$1000 km s$^{-1}$, possibly due to jet-cloud interactions. With the small
number of detections towards cores, the difference in detection rate between
FR\,I (4/32) and FR\,II (3/15) sources is within the statistical uncertainties.
