| Annu. Rev. Astron. Astrophys. 1980. 18:
165-218 Copyright © 1980 by . All rights reserved |
5.3. Angles and Things
All relations between the structure of radio sources and the properties of their associated galaxies and quasars give basic information concerning the formation of sources. During the last few years several statistical relationships have been discovered between gross optical features and radio structures.
One of the most important relations concerns the orientation of double radio sources relative to that of their parent galaxies. Recently, careful studies have been carried out to investigate this, with samples of 17 (Guthrie 1979) and 78 (Palimaka et al. 1979) objects respectively. In each case rigorous selection criteria were applied and only objects with accurately known radio and optical orientation were included. Both studies convincingly show (in contradiction to some earlier work, e.g. Gibson 1975, Sullivan & Sinn 1975) that the radio structures are preferentially but not exclusively aligned along the (projected) minor axes of the elliptical galaxies. This effect appears strongest for the largest radio sources, implying that alignment of the radio-source production axis with the parent galaxy minor axis may favor the development of giant radio sources.
If most radio galaxies rotate as nearly oblate systems, such an alignment would indicate that the radio-source axes are predominantly oriented along the angular momentum axes of the galaxies. The dynamics of five narrow edge-brightened double radio galaxies, 3C33, 3C98, 3C184.1, 3C218, and Cygnus A have been studied (Simkin 1977, 1979) and in all cases the rotation axes of the gas and/or stars do seem indeed to be directed to within a few degrees of the radio structures.
Yet more evidence that the radio axis is related to the rotation axes of galaxies is provided by the dust lanes that are often seen to cross the optical images of radio galaxies. A study of eight radio galaxies with dust lanes (Kotanyi & Ekers 1979, Kotanyi 1979, Ekers et al. 1978c) shows that the seven unambiguous cases have dust lanes perpendicular to the axes of the associated radio sources. But again this alignment is not exclusive (Butcher et al. 1980).
Alignment has also been observed between the extended radio structure and the optical polarization of QSOs. Most QSOs are weakly polarized at the 1% level or less. A comparison of the radio orientation and the optical polarization angles for 24 quasars shows that for 20 of them there is agreement to 30° (Stockman et al. 1979, Angel & Stockman 1980). Although the reason for this alignment is unknown, one can draw the elementary conclusion that the long term memory of direction established for edge-brightened double radio galaxies is also a feature of edge-brightened double quasars.
The apparently simple dynamical behavior of galaxies associated with edge-brightened double sources contrasts with the disturbed optical appearance and complex spectroscopic properties of the few wide extended radio galaxies whose distributions have been studied spectroscopically - e.g. NGC 5128/Centaurus A (Blanco et al. 1975, Osmer 1978, Dufour et al. 1979, Graham 1979), NGC 1316/Fornax A (Schweizer 1979), M87/Virgo A (Young et al. 1978, Sargent et al. 1978), and NGC 1275/3C84 (Burbidge & Burbidge 1965, Rubin et al. 1977). In both Fornax A and Centaurus A, disks of gas and dust are misaligned with the major stellar axes and it has been suggested that the optical properties of these giant elliptical galaxies have been affected by merging. A connection between wide radio sources and galaxy merging would be consistent with Rees's (1978b) suggestion (Section 2.2.3) that merging can cause misalignment between the angular momentum axis of a galaxy and of the nuclear machine responsible for radio emission. The radio ejection axis would then precess, causing a wide radio source to be produced with a wide rotationally symmetric morphology. It may be significant that in Centaurus A the dust lane (Graham 1979) is warped in the same sense as the extended radio emission (Cooper et al. 1965).
Finally, we mention another possible connection between radio and optical morphology which could, if confirmed, be profoundly important. Some years ago it was discovered that two of the brightest tailed radio galaxies, NGC 1265/3C83.1B and NGC 7720/3C465, have asymmetric optical brightness distribution, both having the steepest gradients in the direction of their radio heads (Bertola & Perola 1973). On the radio-trail picture this might imply the existence of a relation between the optical asymmetry and the direction of motion of a galaxy in a rich cluster. Since this effect was reported, the sample of tailed radio galaxies has been enlarged considerably and work is at present in progress to investigate whether there is indeed a statistically significant relation between optical asymmetry and radio morphology (E.A. Valentijn et al., in preparation).