8.2 Possible Complications for Unification
8.2.1 Properties of the Obscuring Torus
The opening angle of the obscuring torus may well be a function of source power. For example, the inner radius of the torus could be determined by the evaporation temperature of dust, Rin ~ 0.06 (Lbol / 1045 erg s-1)0.5 pc (Lawrence 1991; Netzer and Laor 1993). Since more distant quasars are more luminous, this would produce a positive dependence of the quasar fraction on redshift. There are claims in the literature that the ratio of broad- to narrow-line objects depends on redshift (Lawrence 1991; Singal 1993b), but at least in the 3CR sample this trend disappears once the low-excitation FR IIs are excluded (Sec. 8.1.2). The misclassification of some quasars as NLRG could also confuse the issue (Sec. 6.1.1).
An interesting distinction between high- and low-luminosity unified schemes for radio-loud AGN is that the high-luminosity AGN include transition objects, the SSRQ, which are relatively unbeamed in terms of radio emission but ``beamed'' (i.e., unobscured) in terms of optical spectrum. The analog of SSRQ for the low-luminosity AGN - broad-line radio galaxies with FR I morphologies - is not seen (with the exception of the BLRG 3C 120); surveys find either narrow-line FR I or BL Lacs (with occasional broad lines). If FR I galaxies do have broad emission lines at some level, the lack of broad lines could be explained if the opening angle of the torus depended on luminosity (Falcke et al. 1995; Sec. 5.4.2). This would mean the torus opening angle in FR Is is much smaller than in FR IIs. To eliminate broad-line FR Is, it must be comparable to or smaller than the radio beaming angle, whereas in quasars the torus opening angle is much larger than the critical angle for relativistic beaming of the radio emission. Sensitive infrared spectroscopy and spectropolarimetry of FR Is will address this issue by detecting or putting interesting limits on the broad-line luminosities of FR I radio galaxies. ISO observations of radio galaxies, BL Lac objects, and quasars will constrain the relative amounts of obscuring matter in each class.
Direct mapping of the torus may be possible with high spatial resolution spectroscopic observations of water masers. Recent VLBI imaging of water masers in NGC 4258, a nearby galaxy with low-level AGN activity, provides evidence of rotating molecular gas in a region less than 0.13 pc from the center (Miyoshi et al. 1995). This is important direct evidence for molecular gas in the nuclei of galaxies, as implied by the obscuring torus hypothesis. The case of NGC 4258 is not yet the desired demonstration of a molecular torus in AGN, however, because the molecular gas is distributed in a very thin disk rather than a torus. Using the results of Miyoshi et al. (1995) we find the ratio of disk height to inner radius is 0.02, corresponding to an opening angle of ~ 90°, which will clearly not obscure the central source very effectively. In addition, masers do not automatically signal the presence of a torus; in NGC 1068, there are masers in the southern radio component with locations and velocities approximately commensurate with rotation, but there is also a maser to the north that apparently arises in molecular gas shocked by the radio jet (Gallimore et al. 1995).