Infrared polarimetry of the nucleus of Centaurus A - The nearest blazar?

Abstract

As one of the nearest examples of an active galaxy, NGC5128 (Centaurus A) has been studied in detail over a wide range of wavelengths. The nucleus of the galaxy is seen clearly in the X-ray, radio and infrared, but is obscured in the optical by the prominent warped dust lane. We have made polarization observations of the infrared nucleus at wavelengths from 1.2 to 3.8 µm. We find that after correction for the polarization caused by the dust lane, and for dilution by starlight, the nucleus has a large intrinsic polarization of 9% at position angle 147°. This position angle is perpendicular to the direction of the X-ray and radio jet. We interpret the polarized emission from the nucleus as synchrotron radiation from a region whose magnetic field is parallel to the jet direction. The properties of the Cen A nucleus are essentially identical to those of the much more luminous blazars. This suggest that blazar-type activity extends over a very wide range in luminosity, and low-luminosity blazars may be common in elliptical galaxies.