We present the results of H I absorption measurements towards a sample of nearby compact steep-spectrum (CSS) and gigahertz-peaked spectrum (GPS) radio sources, the Compact Radio sources at Low Redshift (CORALZ) sample, using the Giant Metrewave Radio Telescope (GMRT). We observed a sample of 18 sources and find seven new detections. These sources are of lower luminosity than earlier studies of CSS and GPS objects, and we investigate any dependence of H I absorption features on radio luminosity. 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. The relative velocity of the blueshifted absorption features, which may be due to jet-cloud interactions, are within ∼−250 km s −1 and do not appear to extend to values over 1000 km s −1 seen for the more luminous objects. This could be due to the weaker jets in these objects, but requires confirmation from observations of a larger sample of sources. There appears to be no evidence of any dependence of H I column density on either luminosity or redshift, but these new detections are consistent with the inverse relation between H I column density and projected linear size.

We present the results of Giant Metrewave Radio Telescope (GMRT) 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 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 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 blue-shifted components > ∼ 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.

We report the discovery of Hi 21-cm absorption towards the well-studied GHz Peaked-Spectrum source CTA 21 (4C 16.09) using the Arecibo Telescope on 2009 September 20 and 21. Recently, the frequency band between 700 and 800 MHz was temporarily opened up to radio astronomy when US TV stations were mandated to switch from analog to digital transmissions, with new frequency allocations. The redshifted Hi frequency for CTA 21 falls within this band. CTA 21 has a complex radio structure on a range of scales. The innermost prominent components are separated by ∼12 mas while weak diffuse emission extends for up to ∼300 mas. The Hi absorption profile that we find has two main components, one narrow, the other wider and blue-shifted. The total Hi column density is 7.9×10 20 cm −2 , assuming a covering factor of unity and a spin temperature of 100 K. This Hi absorption confirms the recently determined optical redshift of this faint galaxy of z∼0.907. We discuss this new detection in the light of Hi absorption studies towards compact radio sources, and also the possibility that CTA 21 may be exhibiting multiple cycles of nuclear activity. This new detection in CTA 21 is consistent with a strong trend for detection of Hi absorption in radio galaxies with evidence of episodic nuclear/jet activity.

Hi 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 Hi absorption on the nature of radio galaxies based on their emission-line spectra, nature of the host galaxies based on the 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 Hi absorption detections and non-detections. We report a high detection rate of Hi absorption in the galaxies with WISE infrared colours W2−W3 > 2, which is typical of gas-rich systems, along with a compact radio structure. The Hi detection rate for low-excitation radio galaxies (LERGs) with W2−W3 > 2 and compact radio structure is high (70.6±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 Hi 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 Hi detection rates.