The infrared polarizations of high-redshift radio galaxies

Abstract

This thesis reports the K-band polarizations of a representative sample of nine radio galaxies: seven 3C objects at 0.7 < z < 1.3, and two other distinctive sources. Careful consideration is given to the accurate measurement and `debiasing&#39; of faint polarizations, with recommendations for the function of polarimetric software. 3C 22 and 3C 41 both have 3% polarization perpendicular to their radio structures, consistent with suggestions that they may be obscured quasars. 3C 54 is polarized at 6%, parallel to its radio structure. 3C 65 and 3C 356 are faint: the noisy measurements give no firm evidence for polarization. MRC 0156-252 at z~2 is found to be unpolarized in K. 3C 114 has a complex structure of four bright knots, one offset from the radio structure and three along the axis. There is strong evidence for polarization in the source as a whole (12%) and the brightest knot (5%). 3C 441 lies in a rich field; one of its companions appears to be 18% polarized. The identification of the knot containing the active nucleus has been disputed, and is discussed. LBDS 53W091 was controversially reported to have a 40% H-band polarization. No firm evidence is found for non-zero K-band polarization in 53W091, though there is some evidence for its companion being polarized. The object is discussed in the context of other radio-weak galaxies. Simple spectral and spatial models for polarization in radio galaxies are discussed and used to interpret the measurements. The important cosmological question of the fraction of K-band light arising in radio galaxy nuclei is considered: in particular, the contribution of scattered nuclear light to the total K-band emission is estimated to be of order 7% in 3C 22 and 3C 41, 26% in 3C 114, and tentatively 25% or more in 3C 356.