VLT spectropolarimetry of two powerful radio galaxies at z ∼ 1.4:: ultraviolet continuum, emission-line properties and the nature of high-redshift dust

Deep spectropolarimetric observations, obtained with the Very Large Telescope, are presented for two powerful radio galaxies, 0850-206 (z = 1.3373) and 1303 + 091 (z = 1.4093). These observations cover the rest-frame wavelength range 1450-3750 Angstrom. New radio observations and continuum images of the same sources are also presented. These galaxies are the first two observed from a complete sample of nine radio sources with redshifts in the range 1.3 less than or equal to z less than or equal to 1.5 (selected from the equatorial sample of powerful radio sources of Best, Rottgering & Lehnert), as part of a project aimed to investigate the multicomponent nature of the ultraviolet (UV) continuum in radio galaxies and, in particular, any variations of the continuum properties with the radio source age. The larger radio source of the two, 0850-206, presents a high continuum fractional polarization, averaging 17 per cent across the observed wavelength range and reaching 24 per cent at rest-frame wavelengths of less than or similar to2000 Angstrom. The smaller radio source, 1303 + 091, shows a lower continuum polarization, averaging 8 per cent and rising to 11 per cent for rest-frame wavelengths greater than or similar to3000 Angstrom. For both galaxies, the position angle of the electric vector is generally constant with wavelength and within similar to15degrees of perpendicular to the radio axis. Both their total flux spectra and polarized flux spectra reveal the 2200-Angstrom dust feature, and comparison with dust scattering models suggests that the composition of the dust in these galaxies is similar to that of Galactic dust. In 0850-206, scattered quasar radiation dominates the UV continuum emission, with the nebular continuum accounting for no more than similar to22 per cent and no requirement for any additional emission component such as emission from young stars. In contrast, in 1303 + 091, unpolarized radiation could be a major constituent of the UV continuum emission, with starlight accounting for up to similar to50 per cent and the nebular continuum accounting for similar to11 per cent. The emission-line properties of the galaxies are also studied from their total intensity spectra. Comparison of the measured emission-line ratios with both shock- and photoionization models shows that the nuclear and extended gas in these galaxies is mainly photoionized by the central active nucleus.