The central regions of many AGN appear to contain obscuring material, probably in the form of dust, that prevents infrared through ultraviolet light from penetrating some lines of sight (Rowan-Robinson 1977). This dust may be distributed in a torus (Pier and Krolik 1992, 1993) or in a warped disk (Sanders et al. 1989). In either case, it causes AGN to look markedly different from different aspect angles. Direct evidence for obscuration has been found in many Type 2 AGN, although mostly in the more numerous radio-quiet objects. Those NLRG in which obscuration has been detected because of scattered broad emission lines are all FR IIs; detection of such hidden lines in FR Is would provide an intriguing connection between the high- and low-luminosity unification schemes.
3.1 Polarimetric Evidence for Hidden Nuclear Regions
The most direct evidence for circumnuclear obscuration comes from spectropolarimetry of Type 2 objects, particularly the nearby Seyfert 2 NGC 1068 (Antonucci and Miller 1985) and the radio galaxy 3C 234 (Antonucci 1984). Some fraction of the light from these objects is highly polarized, and their polarized spectra have strong broad lines like Type 1 rather than Type 2 objects. Much of the polarization is probably caused by electron scattering since it is wavelength independent; some scattering by dust clouds has also been observed (Miller et al. 1991). In 3C 234, the plane of polarization is perpendicular to the radio jet axis, as expected if a Type 1 nucleus is at least partially obscured by a thick wall of gas and dust whose axis coincides with the radio jet axis. (4) The continuum luminosity inferred from the strength of the scattered broad lines is quasar-like.
Subsequent polarization observations generally support the picture of scattered light from a luminous, hidden continuum source in NLRG. Like 3C 234, the radio galaxy IC 5063 has polarized broad lines, indicative of scattered light from a hidden broad-line region (Inglis et al. 1993). Spectropolarimetry of three powerful radio galaxies at z ~ 1 has shown broad polarized Mg II lines, with equivalent widths typical of those observed in radio-loud quasars (di Serego Alighieri et al. 1994a). Multiwavelength polarimetry of 3C 109 (Goodrich and Cohen 1992) suggests polarization by transmission through dust, in which case the intrinsic luminosity is also quasar-like. (5)
Recent HST spectra of Cygnus A show a broad Mg II emission line which appears to be reflected from the southeast knot (a region of extended featureless optical continuum; Pierce and Stockton 1986). This is a direct signature of the hidden quasar in this luminous FR II galaxy (Antonucci et al. 1994); that broad emission lines have not been detected with optical spectropolarimetry (Jackson and Tadhunter 1993) may be because of dilution by the local optical continuum at the scattering site (Antonucci et al. 1994; Sec. 8.2.6).
Imaging polarimetry of the radio galaxies PKS 2152-69
(di Serego Alighieri
et al. 1988),
(di Serego Alighieri
et al. 1989),
(di Serego Alighieri
et al. 1989;
Scarrott et al. 1990),
and additional radio galaxies
(Cimatti et al. 1993,
and references therein)
reveals extended regions of polarized continuum emission which appear to be
nuclear continuum scattered by ambient dust and/or electrons.
Tadhunter et al.
(1989; see also
Cimatti et al. 1993 and
di Serego Alighieri
et al. 1994a)
have suggested that
dust-scattering of quasar radiation could also explain the so-called
``alignment effect'' observed in high-redshift radio galaxies
(McCarthy et al. 1987;
Chambers et al. 1987),
i.e., the correlation between the direction of the
radio axis and the optical structure (extended emission line regions plus
5 Like 3C 234, 3C 109 has strong
broad lines observable in total flux, in this case H. While the
spectral shapes of the
polarized continuum in 3C 109 and 3C 234 are similar, the polarization in
3C 234 is too high to be caused by dust transmission
(Goodrich, private communication). Back.
4 Antonucci (1984) identified 3C 234 as a narrow-line radio galaxy even though it has broad H (Grandi and Osterbrock 1978). His point was that 3C 234 is equivalent to a narrow-line object like NGC 1068 because its broad lines are very highly polarized and therefore must be scattered. The broad emission lines of 3C 234 were not really ``hidden'' prior to the spectropolarimetric observations (although they were not strong broad lines that would clearly mark it as a quasar), but spectropolarimetry indicated that a direct view to the broad-line region was obscured. The fuzzy definition of ``broad-line'' (Type 1) objects is discussed further in the footnote in Sec. 6.1.1. Back.
5 Like 3C 234, 3C 109 has strong broad lines observable in total flux, in this case H. While the spectral shapes of the polarized continuum in 3C 109 and 3C 234 are similar, the polarization in 3C 234 is too high to be caused by dust transmission (Goodrich, private communication). Back.