H I 21 cm ABSORPTION AND UNIFIED SCHEMES OF ACTIVE GALACTIC NUCLEI

Abstract

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 L UV ∼ 10 23 W Hz −1 . 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 L UV < ∼ 10 23 W Hz −1 , 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 10 23 W Hz −1 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 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 < ∼ 10 23 W Hz −1 ) 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 colour, 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.

Atomic and molecular absorption in redshifted radio sources

“…However, as well as occuring in outflowing gas (e.g. Vermeulen et al 2003;Morganti et al 2003Morganti et al , 2005aMorganti et al ,b, 2007Allison et al 2016), the bulk of the absorption is believed to occur in the large-scale galactic disk, rather than the sub-pc torus 13 , which is expected to have a random orientation to the radio jets (Curran & Whiting 2010 and references therein).…”

“…A decrease in the detection rate with redshift was first shown by Curran et al (2008), which was interpreted as the high redshifts selecting UV luminosities sufficient to excite the gas to below the detection limit. 10 Be- cause of the Malmquist bias, it can be difficult to ascertain whether the decreasing detection rate is caused by an evolutionary effect at high redshift (Curran & Whiting 2010;Aditya et al 2016), alluminosity in their comparison with the highest redshift (z = 3.40) H I 21-cm detection (Uson et al 1991). This, B2 0902+34, has L UV ≈ 3 × 10 22 W Hz −1 , cf.…”

Correlation between X-Ray and Radio Absorption in Compact Radio Galaxies

“…This finding may indicate that the typical crosssection of cold, absorbing gas is 0.1−1 kpc, in "resonance" with the radio source size. The detection rate of HI absorption was also found to be partly affected by the UV luminosity of the AGN: in sources with L UV > 10 23 W Hz −1 , mostly extended, a larger fraction of the hydrogen reservoir may be ionized, decreasing the likelihood of HI detection (Curran & Whiting 2010;Allison et al 2012). The statistics of HI detections thus seem to suggest that compact radio sources are hosted by systems that are, on average, richer in neutral gas than extended sources and that this gas is mostly concentrated in structures with typical linear size of 0.1−1 kpc, i.e., larger than the pc-scale dusty tori required by unification schemes (Antonucci 1993;Urry & Padovani 1995;Tadhunter 2008) and recently imaged in nearby Seyfert galaxies (e.g., Jaffe et al 2004;Raban et al 2009;Tristram et al 2009Tristram et al , 2014.…”

The Geometry Effects of an Expanding Universe on the Detection of Cool Neutral Gas at High Redshift

“…where T spin (K) is the mean harmonic spin temperature of the gas, σ is the depth of the line (or rms noise in the case of a non-3 Summarized in Curran & Whiting (2010), with the addition of three new associated absorbers reported in Curran et al (2011aCurran et al ( , 2011c. 4 Summarized in Curran (2010), with the addition of those reported by Curran et al (2011b) and Srianand et al (2012).…”