A multiband study of Hercules A. II. Multifrequency VLA imaging

Abstract

We have mapped the powerful radio galaxy Hercules A at six frequencies spanning 1295 to 8440 MHz using the VLA in all four configurations. Here we discuss the structure revealed in total intensity, spectral index, polarization, and projected magnetic field. Our observations clearly reveal the relation between the bright jets, radio source. The jets and rings form a coherent structure with a dramatically flatter spectrum than the surrounding lobes and bridge, strongly suggesting that they represent a recently renewed outburst from the active nucleus. The spectrum of the lobes is also steeper than in typical radio sources, and steepens further towards the centre. The compact core is optically thin and also has a remarkably steep spectrum (alpha \~ -1.2). There is some evidence that the old lobe material has been swept up and compressed ahead of the new outburst. We interpret the dramatic asymmetry in the bright structure, and more subtle differences between diffuse lobe structures, in terms of relativistic beaming combined with front-to-back light-travel delays which mean that we view the two lobes at different stages of the outburst. After correcting for Faraday rotation the projected magnetic field closely follows the edge of the lobes, the jets, and the rings; the field pattern in the two lobes is broadly similar. We confirm a strong asymmetry in depolarization and Faraday rotation, with the jet side the less depolarized and the flatter spectrum, consistent with general correlations between these asymmetries. The spectral index asymmetry is clearly present in the `old' lobe material and so, at least in this case, is not due to beaming; but it can be understood in terms of the light-travel delay.Comment: 25 pages, 18 figures, accepted for publication in MNRAS. For associated jpeg files, see http://www.jb.man.ac.uk/~jpl/hera