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

Abstract

A search for HI absorption has been made in 23 radio galaxies using the ATCA, the VLA and the 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% of total) was detected. The HI absorption in these objects is likely to come from a nuclear disk, as found for other galaxies of this type. The low detection rate is consistent with the hypothesis (as suggested by optical and X-ray data) that the ``standard'' pc scale, geometrically thick torus is not required in low-luminosity radio galaxies. 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 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 due to 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 that the HI is blueshifted compared to 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, we find a tendency for radio galaxies with a strong component of young stellar population and far-IR emission to show HI absorption. The overall richer ISM that is likely to be present in these galaxies may be a factor in producing the absorption.