NGC 326: X-shaped no more

Hardcastle, M J; Croston, J H; Shimwell, T W; Tasse, C; Gürkan, G; Morganti, R; Murgia, M; Röttgering, H J A; van Weeren, R J and Williams, W L (2019). NGC 326: X-shaped no more. Monthly Notices of the Royal Astronomical Society, 488(3) pp. 3416–3422.



We present new 144-MHz LOFAR observations of the prototypical ‘X-shaped’ radio galaxy NGC 326, which show that the formerly known wings of the radio lobes extend smoothly into a large-scale, complex radio structure. We argue that this structure is most likely the result of hydrodynamical effects in an ongoing group or cluster merger, for which pre-existing X-ray and optical data provide independent evidence. The large-scale radio structure is hard to explain purely in terms of jet reorientation due to the merger of binary black holes, a previously proposed explanation for the inner structure of NGC 326. For this reason, we suggest that the simplest model is one in which the merger-related hydrodynamical processes account for all the source structure, though we do not rule out the possibility that a black hole merger has occurred. Inference of the black hole-black hole merger rate from observations of X-shaped sources should be carried out with caution in the absence of deep, sensitive low-frequency observations. Some X-shaped sources may be signposts of cluster merger activity, and it would be useful to investigate the environments of these objects more generally.

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