Quasar Absorption Lines and Radio Polarization

Abstract

Conway and Gilbert¹ have recently reported that the mean linear polarization at 49 cm is lower for quasars with absorption-line spectra than for those with pure emission-line spectra. Although the sample size is small, they “believe that the depolarization is due to Faraday rotation in the same regions as are responsible for the absorption lines”. To examine the effect in more detail we list in Table 1 the mean values of percentage polarization m̄ at 11 cm (ref. 2 and unpublished work of Whiteoak, Gardner and Morris) and 6 cm (ref. 3) for the two classes of quasars with z > 1.25, together with those at 49 cm (ref. 1). The sample sizes given in parentheses are small, but it is notable that the differences are comparable at all wavelengths. At 11 cm and 6 cm a consideration of median values, less influenced by individual high polarizations, leads to the same result. We conclude that the polarization difference is due to a lower intrinsic polarization for the absorption-line sources rather than to a larger Faraday depolarization because the latter would increase by at least λ², that is, by sixty-six times between 6 and 49 cm wavelengths. The difference would contribute in part to the observed decrease of polarization with redshift¹, because the relative abundance of absorption-line sources for redshifts exceeding 1.25 is about seven times that at lower redshifts⁴.