H I 21 - cm absorption and unified schemes of active galactic nuclei

In a recent study of z > ∼ 0.1 active galactic nuclei (AGN), we found that 21-cm absorption has never been detected in objects in which the ultra-violet luminosity exceeds LUV ∼ 10 W Hz. In this paper, we further explore the implications that this has for the currently popular consensus that it is the orientation of the circumnuclear obscuring torus, invoked by unified schemes of AGN, which determines whether absorption is present along our sight-line. The fact that at LUV < ∼ 10 W Hz, both type-1 and type-2 objects exhibit a 50% probability of detection, suggests that this is not the case and that the bias against detection in type-1 objects is due purely to the inclusion of the LUV > ∼ 10 W Hz sources. The inclusion of these sources also gives the apparently high 21-cm detection rate in compact objects, such as CSSs and GPSs. We find that ultra-violet luminosities can also explain why the presence of 21-cm absorption shows a preference for radio galaxies over quasars. Examining the LUV < ∼ 10 W Hz sample further, from the profile widths and offsets from the systemic velocities, we find no discernible differences between the two AGN types. If the galactic disk shares a similar orientation to the central obscuration, this would suggest that the outflows of cold neutral gas, which would give rise to the absorption in type-1 AGN, have similar apparent kinematics to the toroidal gas. However, the fact that only half of the sources are detected in 21-cm, regardless of AGN type, may suggest that the bulk absorption generally occurs in the disk, which must therefore be randomly orientated with respect to the circumnuclear torus. Furthermore, we see no difference in the reddening between the two AGN types, indicating, like the 21-cm absorption, that the orientation of the torus has little bearing on this. It therefore appears that both the reddening and bulk H I are located in the disk, which is supported by the correlation between 21-cm line strength and the optical–nearinfrared colour, thus suggesting that the reddening is caused by dust located in the large-scale, H I absorbing disk which intervenes the sight-line to the AGN.