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6th of April 2023
Potential HI Compass Needles in the SMC
by Ma et al.
High-spatial-resolution HI (neutral hydrogen) observations within the Milky Way have led to the realisation that the nearby atomic filamentary structures are aligned with the ambient Galactic magnetic field. Ma et al. have used data from the Galactic ASKAP HI (GASKAP-HI) survey, to investigate the potential magnetic alignment of the > 10 pc-scale HI filaments in the Small Magellanic Cloud (SMC). A Rolling Hough Transform (RHT) technique was used to automatically identify filamentary structures in the ASKAP data, and this was combined with a newly devised ray-tracing algorithm that compares the HI and starlight polarisation data. The results indicate that the HI filaments in the north-eastern end of the SMC main body (the "Bar" region) and the transition area between the main body and the tidal feature (the "Wing" region) appear preferentially aligned with the magnetic field traced by starlight polarisation. The remaining SMC volume lacks starlight polarisation data of sufficient quality to draw any conclusions. This suggests for the first time that filamentary HI structures can be magnetically aligned across large spatial volumes outside of the Milky Way.

The figures above illustrate the automatically identified filaments using the RHT. The left panel shows a zoomed-in image of the central area of the GASKAP-HI SMC map, and the right panel shows the corresponding RHT back-projection map, where any non-zero pixels are regarded as a filament. The "Bar" of the SMC runs from the top-centre to the bottom-right of the images, with the "Wing" extending from the centre to the bottom-left. (Compare with a full ASKAP image of the SMC.) The team generate maps of the preferred orientation of HI filaments in the SMC, revealing the highly complex gaseous structures of the galaxy likely shaped by a combination of the intrinsic internal gas dynamics, tidal interactions, and star formation feedback processes. These maps can further be compared with future measurements of the magnetic structures in other regions of the SMC.

As Friday April 7th and Monday April 10th are public holidays in Australia, the next ADAP will be Tuesday April 11th.

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