Infrared spectropolarimetry of Seyfert galaxy NGC1068

Abstract

Although most of the luminosity of Seyfert galaxy nuclei is emitted in the IR, the relative amounts of thermal dust and non-thermal emission are uncertain. Seyfert galaxies generally have smooth and featureless IR spectra, in contrast to the prominent dust emission features seen in galaxies whose nuclei resemble giant H II regions—the star burst nuclei^1,2. Nevertheless, in type 2 Seyferts, the general form of the energy distribution and lack of marked variability suggest that here too thermal dust emission is dominant^3–5, and in NGC1068 this supposition is supported by the resolved size of its nucleus⁶. Dust is observed in absorption at 10 µm (refs 7,8) and in emission near 20 µm (ref. 9). Polarization has been detected from the UV to 10 µm, and attributed to scattering and absorption at short wavelengths¹⁰ with a possible non-thermal component^11,12, to scattering and non-thermal radiation in the near IR⁹, and to either absorption or non-thermal emission in the 10 µm region¹³. We present here spectropolarimetric measurements of NGC1068, between 8 and 13 µm, which show that the polarized flux at these wavelengths is intrinsic to the emission mechanism and not due to absorption by aligned grains.