4.2. Photoionization by the Nucleus: Narrow Angle Collimation by a Relativistic Jet
A quite different form of radiation anisotropy is the narrow cone emitted by a relativistic jet. Morganti et al. (1991, 1992) have recently investigated the line-emitting filaments (Fig. 3) which compose part of the "optical jet" of Centaurus A. From an analysis of the line intensity ratios, they conclude that the filaments are photoionized predominantly by the radiation field of a nuclear continuum source which is hidden from our direct view by either obscuration, or intrinsic anisotropy, or both. The radiation field in the 2-6 kev band required to account for the ionization state of the gas is 200 times stronger than that emitted towards Earth. If this is the case, Cen A has a beam power similar to that of a BL Lac object, consistent with "unified models" in which FRI radio galaxies (like Cen A) are the parent population of BL Lacs (e.g., Urry, Padovani & Stickel 1991). Observed ionization gradients within the individual filamentary structures in Cen A support the notion that the ionizing photons arrive from more or less the direction of the galaxy nucleus (see Fig. 3 and Morganti et al. 1992). Nevertheless, the arguments for photoionization of the Centaurus A filaments by radiation from a nuclear relativistic beam, although persuasive, are not totally compelling. Alternative models, in which shocks play a role in the ionization, deserve investigation (see remarks by Bicknell in the discussion following this paper).
Figure 3. (from Morganti et al. 1992). The inner optical emission-line filament system 9 arc min ( 9 kpc) north-east of the nucleus of Centaurus A (NGC 5128). The images are constructed from [OIII]5007 and H + [NII]6548, 6583 narrow-band filter observations. The image to the upper right shows the [OIII] intensity image with various regions marked A-F. The images in the center and lower left show, at the same scale and an enlarged scale, respectively, the flux ratio image [OIII] / H (the ratio has been scaled to exclude [NII], as the ratio [NII] / H is believed to be almost constant). The ratio images are shown as gray scales, with light gray representing R 1, and black representing R 4. The apparent direction of the nuclear radiation field is shown with an arrow. The light region at the southern edge of knot A has been shown to be like a stellar photoionized HII region.
Another possible case of a "misdirected" beam of ionizing radiation resulting from a relativistic jet is PKS2152-69. This powerful radio galaxy has both a highly ionized emission-line cloud and an associated source of scattered optical and ultraviolet continuum light close to the radio axis (Tadhunter et al. 1987, 1988; di Serego Alighieri et al. 1988). A narrow, elongated, emission-line feature in the Seyfert 1 galaxy NGC 3516 may represent another example of gas photoionized by radiation from a relativistic, nuclear jet (Miyaji, Wilson, & Pérez-Fournon 1992).