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HI 21 cm Absorption and Unified Schemes of Active Galactic Nuclei

S.J. Curran and M.T. Whiting

In a recent study of z ≥ 0.1 active galactic nuclei (AGNs), we found that 21 cm absorption has never been detected in objects in which the ultraviolet luminosity exceeds L_UV ~ 10²³ 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 AGNs, which determines whether absorption is present along our sight line. The fact that at L_UV ≤ 10²³ 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 of H I absorption in type-1 objects is due purely to the inclusion of the L_UV ≥ 10²³ W/Hz sources. Similarly, the ultraviolet luminosities can also explain why the presence of 21 cm absorption shows a preference for radio galaxies over quasars and the higher detection rate in compact sources, such as compact steep spectrum or gigahertz peaked spectrum sources, may also be biased by the inclusion of high-luminosity sources. Being comprised of all 21 cm searched sources at z≥0.1, this is a necessarily heterogeneous sample, the constituents of which have been observed by various instruments. By this same token, however, the dependence on the UV luminosity may be an all encompassing effect, superseding the unified schemes model, although there is the possibility that the exclusive 21 cm non-detections at high UV luminosities could be caused by a bias toward gas-poor ellipticals. Additionally, the high UV fluxes could be sufficiently exciting/ionizing the H I above 21 cm detection thresholds, although the extent to which this is related to the neutral gas deficit in ellipticals is currently unclear. Examining the moderate UV luminosity (L_UV ≤ 10²³ W/Hz) sample further, from the profile widths and offsets from the systemic velocities, we find no discernible differences between the two AGN types. This may suggest that the bulk of the absorption generally occurs in the galactic 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. We also find a correlation between 21 cm line strength and the optical-near-infrared color, which suggests that the reddening is caused by dust located in the large-scale, H I absorbing disk which intervenes the sight line to the AGN.

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