Linear Polarization of Distant Quasars

Abstract

MEASUREMENTS of the linear polarization of quasars at a wavelength of 49 cm (m₄₉) have already shown that this parameter is correlated with spectral index α (ref. 1). We have now observed a more complete sample of 93 quasars, all of which have measured redshifts. The correlation between m₄₉ and α is confirmed, but a further correlation has been detected, namely between m₄₉ and redshift z. The relation between m₄₉ and z is shown in Fig. 1. The redshifts are taken from Burbidge and Burbidge², Schmidt³ and Hiltner et al.⁴. The spectral indices represent the spectral slope at or near 49 cm. The separation between flat spectrum and steep spectrum quasars is especially marked for low redshifts (z < 1.25). This separation does not exist for polarization measured at shorter wavelengths, and is therefore a depolarization effect, presumably by Faraday rotation within the source. Fig. 1 shows that the median value of m₄₉ falls markedly as z increases for flat spectrum quasars. The median value of m₄₉ is almost independent of z for steep spectrum quasars.