3.3 Anisotropic Illumination of Narrow Emission Line Gas
The presence of anisotropic continuum emission can be inferred from the extended narrow line regions, sometimes tracing ionizing light cones, seen in direct imaging of many nearby Type 2 AGN. High-resolution HST images of NGC 1068 in the light of [O III] (Evans et al. 1991) confirm in exciting detail earlier ground-based evidence for an ionization cone with apex at an obscured nucleus (Pogge 1988). Fig. 3 shows the HST image of another nearby Seyfert 2 galaxy, NGC 5728 (Wilson et al. 1993); the conical shape of the ionized gas and its filamentary structure are both apparent. In these and other cases, the bi-conical structure suggests that an obscured nuclear source with quasar-like luminosity is photoionizing gas in the extended narrow-line region (Robinson et al. 1987; Baum and Heckman 1989; Tadhunter and Tsvetanov 1989; Wilson et al. 1993).
|Figure 3. HST WFPC image of the nearby Seyfert 2 galaxy NGC 5728 taken in the light of [O III] with resolution 0.1 arcsec (Wilson et al. 1993). The bi-conical structure suggests that a hidden nuclear source with quasar-like ultraviolet luminosity is photoionizing gas in the narrow-line region. (Copyright American Astronomical Society, reprinted with permission.)|
At present, there is no direct evidence (i.e., from spectropolarimetry) for obscuration in low-luminosity, FR I radio galaxies, although there are strong indications of anisotropic continuum emission in some individual objects. (Indirect evidence for obscuration in FR Is as a class is discussed in Sec. 5.4) The FR I radio galaxy PKS 2152-69 shows optical line emission from a gas cloud at a projected distance of 8 kpc from the nucleus which, if due to excitation from a beamed nuclear source, implies an ionizing beam power well within the range for BL Lacs (di Serego Alighieri et al. 1988).
Similarly, in the FR I radio galaxy Cen A, optical emission-line filaments subtend a small projected solid angle in the northwest part of the galaxy, roughly aligned with the position of the jet-like structures found in the X-ray and radio bands (Morganti et al. 1991). A detailed analysis of the line ratios in the Cen A filaments suggests they are photoionized by strongly anisotropic radiation, about two orders of magnitude larger in the X-ray band than that observed directly (Morganti et al. 1992). Were this due to simple obscuration of an isotropic continuum by a thick torus, the obscured radiation would be reradiated in the infrared, contrary to observation. At least some of the continuum emission must be obscured, however, as the variable (i.e., nuclear) X-ray source in Cen A is heavily absorbed (Mushotzky et al. 1978). The continuum anisotropy in Centaurus A could be caused by relativistic beaming (see Sec. 4) - its inferred beam power is similar to that of BL Lacertae (Morganti et al. 1991) - or the optical filaments could be excited by local shocks where the northern radio jet interacts with a dense cloud of material (Sutherland et al. 1993). We note that illuminated clouds like those seen in Cen A and PKS 2152-69 are expected if FR Is found in cooling flow clusters contain hidden BL Lacs (Sarazin and Wise 1993).