HI 21-cm absorption and unified schemes of active galactic nuclei

TL;DR

This paper investigates the factors influencing 21-cm HI absorption in active galactic nuclei, challenging the orientation-based unified scheme and highlighting the role of UV luminosity and source properties in detection biases.

Contribution

It demonstrates that UV luminosity and source characteristics significantly affect HI absorption detection, questioning the primacy of orientation in unified AGN models.

Findings

Detection probability is not solely orientation-dependent.

High UV luminosity correlates with lower HI absorption detection.

Sample heterogeneity and luminosity biases influence results.

Abstract

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 which determines whether absorption is present along our sight-line. The fact that at logL > 23 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 HI absorption in type-1 objects is due purely to the inclusion of the logL > 23 W/Hz sources. Similarly, the ultra-violet 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 CSS or GPS 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 towards gas-poor ellipticals. Additionally, the high UV fluxes could be sufficiently exciting/ionising the HI above 21-cm detection thresholds, although the extent to which this is related to the neutral gas deficit in ellipticals is currently unclear.