Radio continuum evidence for nuclear outflow in the Circinus galaxy

Abstract

Australia Telescope Compact Array radio continuum maps of the edge-on galaxy in Circinus are presented for four frequencies. The unresolved core of the galaxy measures less than 20 pc in diameter and has a flat spectral index of a = -0.06 ± 0.15 (S ∝ vα). An arcsecond-scale jet close to the core is directed similarly to the previously detected larger scale plumes. The core is surrounded by what we believe to be a diffuse radio counterpart to the recently discovered r = 200 pc starburst ring. The large radio lobes are shown to consist of central plume features surrounded by edge-brightened emission. The polarized emission from these lobes is mainly contained within these distinct plume and edge-brightened features, the latter showing very high degrees of polarization (> 50 per cent at λ = 6 cm). A study of the depolarization and rotation measures indicates that with respect to the galactic disc, the northern radio lobe is in the foreground and directed towards us whilst the opposite is true for the southern lobe. Analysis of the spectral indices along the plume structures shows slight steepening of values away from the core (α ≃ -0.8), indicative of ageing of electrons along the flow. Brighter hotspots of emission present in both north and south plumes show significantly flatter spectral indices (α ≃ -0.4). This is similar to the spectral index behaviour observed in higher energy astrophysical jets. High field strengths in the edge-brightened regions (up to 18 μG) of the radio lobes are probably owing to compression of the frozen halo field, indicative of interaction between the ejecta driven bow shock with the ambient halo medium. Comparison with other galaxies displaying similar features suggests that although the nuclear starburst is no doubt contributing significantly to the concentration of cosmic rays in the radio lobes, a Seyfert nucleus is needed to explain the collimated jet observed in these objects.