A Polarizing Window on High Redshift Radio Galaxies
We have presented recent imaging polarimetry observations made with EFOSC, which show that the rest frame ultraviolet light of two high redshift radio galaxies, 3C 277.2 and 3C 368, is strongly linearly polarized with E-vectors close to perpendicular to the radio axis (see Table 1). For 3C 368 we have evidence that the polarization is extended and decreases with wavelength. A detailed description of the observations and of the stringent scrutiny of the data, on which the soundness of the results is based, have been presented elsewhere (di Serego Alighieri et al. 1989a and b, Fosbury et al., 1989).
Our observations bear on the problem of understanding the excess of UV light found in high redshift radio galaxies (Lilly & Longair, 1984) and the alignment of their optical major axis, both in the continuum and in the emission lines, with the radio axis (McCarthy et al., 1987, Chambers et al., 1987). These two facts are generally explained with recent star formation (Dunlop et al., 1989), possibly associated with the radio jet (Rees, 1989; De Young, 1989 and this volume). Our results on the other hand imply that a large fraction of the ultraviolet light from these objects does not come directly from stars, although we cannot exclude some contribution from stellar light especially in the K-band (rest frame optical — near IR), where alignments with the radio have also been observed for a few objects (Chambers et al., 1988, Rawlings and Eales, 1989).
We suggest that the polarization is due to scattering of beamed nuclear radiation, probably by dust, a model already proposed by us for a nearby radio galaxy, PKS 2152–69 (di Serego Alighieri et al., 1987). If polarization is a general phenomenon in high redshift radio galaxies and if it is indeed a sign of beamed radiation from the nucleus, this would provide strong support to the so called “unified schemes” seeking to explain the different classes of active galactic nuclei by radiation anisotropies and orientation effects (e.g. Jackson et al., 1989). In addition the fact that elongation and alignment are not seen so strongly at low redshift may tell us something about the evolution of the interstellar medium of the galaxies hosting the active nuclei.
KeywordsRest Frame Active Galactic Nucleus Radio Galaxy Nuclear Radiation Radiation Anisotropy
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