Associated 21-cm absorption towards the cores of radio galaxies

We present the results of Giant Metrewave Radio Telescope observations to detect HI in absorption towards the cores of a sample of radio galaxies. From observations of a sample of 16 sources, we detect HI in absorption towards the core of only one source, the Fanaroff-Riley type II (FR II) radio galaxy 3C 452 which has been reported earlier by Gupta & Saikia. 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 gigahertz peaked-spectrum (GPS) sources. We also compile available information on HI 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 HI absorption towards their cores is significantly smaller (7/47) than the fraction of HI detection towards CSS and GPS objects (28/49). For the cores of extended sources, there is no evidence of a significant correlation between HI 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 blueshifted components greater than or similar to 1000 km s(-1), possibly due to jet-cloud interactions. With the small number of detections towards cores, the difference in the detection rate between FR I (4/32) and FR II (3/15) sources is within the statistical uncertainties.