HI absorption in radio galaxies: effect of orientation or interstellar medium?

A search for HI absorption has been made in 23 radio galaxies selected from a complete sample. The observations were made with the Australia Telescope Compact Array (ATCA), with the Very Large Array (VLA) and for one galaxy with the Westerbork Synthesis Radio Telescope (WSRT). In five galaxies HI absorption was detected. We investigate how the detection rate is distributed among galaxies with different radio and optical properties. Among the Fanaroff-Riley (FR) type I radio galaxies, only one object (10 per cent of the total) was detected in HI absorption. The HI absorption in these objects is likely to come from a nuclear disc, as found for other galaxies of this type (e.g. NGC 4261 and Hydra A). The low detection rate is consistent with the hypothesis (as suggested by optical and X-ray data) that the 'standard' parsec-scale, geometrically thick torus is not required in low-luminosity radio galaxies. This is consistent with earlier optical work. In the case of FR type II powerful radio galaxies, no HI absorption has been detected in broad-line radio galaxies, while three out of four narrow-line radio galaxies have been detected (the one non-detection having quite a high upper Limit). All of these are compact or small radio galaxies. To first order this is consistent with the predictions of the unified schemes, assuming that the HI absorption is caused by an obscuring torus. However, the indications of this being the only cause of the absorption are not very strong. In particular, we find that in two of the three detected objects the HI is blueshifted compared with the systemic velocity. In the third galaxy (PKS 1549-79) two redshift systems (from the optical lines) are found. The uncertainty in the systemic velocity derived from optical lines is discussed. Finally, by also considering data available in the literature, we find a tendency for radio galaxies with a strong component of young stellar population and far-infrared emission to show HI absorption. The overall richer interstellar medium that is Likely to be present in these galaxies may be a factor in producing the absorption.