Imaging a ring-like structure and the extended jet of M87 at 86 GHz

Abstract: The galaxy M87 is one of the prime targets for high resolution radio imaging to investigate the supermassive black hole, accretion flow, and relativistic jet. However, it remains challenging to observe them jointly. In 2018, GMVA+ALMA observations at 86 GHz enabled the simultaneous reconstruction of a ring structure and the extended jet emission. In order to analyze the ring and jet of M87, conventional 'CLEAN' algorithms were mainly employed alongside the RML method 'SMILI' in previous work. To test the robustness of the reconstructed structures of M87 GMVA+ALMA observations at 86 GHz, we estimate the ring diameter, width, and the extended jet emission with the possible central spine by two different novel imaging algorithms: 'resolve' and 'DoG-HiT'. Overall, reconstructions are consistent with the 'CLEAN' and 'SMILI' images. The ring structure of M87 is resolved at a higher resolution and the posterior distribution of M87 ring features is explored. The 'resolve' images show that the ring diameter is 60.9 +- 2.2 muas and its width is 16.0 +- 0.9 muas. The ring diameter and the ring width measured from the 'DoG-HiT' image are 61.0 muas and 20.6 muas, respectively. The ring diameter is therefore in agreement with the estimation in the previous work (64 +4 -8 muas). Two bright spots in the ring are reconstructed by four independent imaging methods. Therefore, the substructure in the ring most likely results from the data. A consistent limb-brightened jet structure is reconstructed by 'resolve' and 'DoG-HiT', albeit with a less pronounced central spine. Modern data-driven imaging methods confirm the ring and jet structure in M87, and complement traditional VLBI methods with novel perspectives on evaluating the significance of the recovered features. They confirm the result of the previous report.