3.2 Infrared and X-Ray Evidence for Hidden Nuclear Regions
Since optical depth decreases at wavelengths longer than the optical, infrared observations are potentially much deeper probes of the nuclear regions of Type 2 AGN. Compact, bright, infrared cores and/or wavelength-independent perpendicular polarization as in 3C 234 have been found in about a dozen narrow-line radio galaxies, of both high and low luminosities (Bailey et al. 1986; Fabbiano et al. 1986; Hough et al. 1987; Antonucci and Barvainis 1990; McCarthy et al. 1990; Djorgovski et al. 1991). In a few cases, infrared spectroscopy has revealed broad wings on the Paschen lines (Fabbiano et al. 1986; Hill et al. 1995), at least partially revealing the infrared high-velocity gas that is completely obscured at optical wavelengths. In others the hidden infrared-optical continuum source and broad-line region remain stubbornly hidden suggesting the expected optical depths are quite large; for example, up to 25-50 magnitudes of visual extinction to the nucleus are estimated in the case of Cygnus A (Ward et al. 1991; Djorgovski et al. 1991).
The weakness of the X-ray continuum from Type 2 AGN relative to Type 1s is also consistent with the idea of an obscured nucleus (Lawrence and Elvis 1982; Fabbiano et al. 1984; Fabbiano et al. 1986). The X-ray spectrum of Cygnus A, a classic high-luminosity narrow-line radio galaxy, is commensurate with a typical quasar spectrum absorbed by a high column density of cold gas along the line of sight (Arnaud et al. 1987; Ward et al. 1991; Ueno et al. 1994). Similar high column densities are deduced from X-ray-measurements of a number Type 2 AGN, exactly as expected if the central source in Type 2s is obscured from direct view (Mulchaey et al. 1992).