Near-infrared Hubble Space Telescope polarimetry of a complete sample of narrow-line radio galaxies

Abstract

We present an analysis of 2.05 $\mu$m Hubble Space Telescope (HST) polarimetric data for a sample of 13 nearby Fanaroff-Riley type II (FRII) 3CR radio sources ($0.03<z<0.11$) that are classified as narrow line radio galaxies (NLRG) at optical wavelengths. We find that the compact cores of the NLRG in our sample are intrinsically highly polarised in the near-IR ($6 < P_{2.05\mu m} < 60$ per cent), with the electric-vector (E-vector) perpendicular to the radio axis in 54 per cent of the sources. The levels of extinction required to produce near-infrared polarisation by the dichroic extinction mechanism are consistent with the measured values reported in Ram\'irez et al. (2014), provided that this mechanism has its maximum efficiency. This consistency suggests that the nuclear polarisation could be due to dichroic extinction. In this case, toroidal magnetic fields that are highly coherent would be required in the circumnuclear tori to align the elongated dust grains responsible for the dichroic extinction. However, it is not entirely possible to rule out other polarisation mechanisms (e.g. scattering, synchrotron emission) with our observations at only one near-IR wavelength. Therefore further polarimetry observations at mid-IR and radio wavelengths will be required to test whether all the near-IR polarisation is due to dichroic extinction.