Research Byte
Published in the RSAA Lunations
Vol1 Issue34 1–30 November 2022
HI filaments and the magnetic field of the SMC: Potential compass needles detected by the GASKAP-HI survey
The Australian Square Kilometre Array Pathfinder (ASKAP) is an excellent surveying telescope at radio wavelengths, with its Phased Array Feed (PAF) receivers granting a wide instantaneous field-of-view of about 30 square degrees (Hotan et al. 2021). One of the many ongoing surveys using the ASKAP is the Galactic ASKAP HI (GASKAP-HI; Dickey et al. 2013) survey, which will provide a view with unprecedented clarity into part of the southern Galactic Plane, the Galactic Centre, and the Magellanic System.
We have observed the Small Magellanic Cloud (SMC) as part of the pilot survey of GASKAP-HI. The resulting data quality is exceptional, with an angular resolution of 30" and surface brightness sensitivity of 1.1 K at a velocity resolution of 0.98 km/s (Pingel et al. 2022). It is immediately apparent that the HI structures of the SMC is highly filamentary, prompting us to investigate in detail the physical properties of these structures, particularly their orientations in the plane-of-sky.
We applied the Rolling Hough Transform (RHT; Clark et al. 2014) algorithm to the GASKAP-HI data of the SMC to automatically identify filamentary structures, and subsequently extracted the preferred orientation of these filaments across the entire SMC. This is shown in the figure as the flow-line texture produced by the Line Integral Convolution algorithm, overlaid on the optical map from the DSS2 survey (Lasker et al. 1996). From this, we identify three distinct regions across the main body of the SMC: (1) in the northeastern end, the filaments are preferentially oriented along the gaseous velocity gradient of the galaxy (Di Teodoro et al. 2019), meaning that the filaments can be following the gas dynamics; (2) around the mid-point of the galaxy, the preferred orientation of the filaments switch to northwest-southeast along the tidal structure formed from the interactions with the Large Magellanic Cloud (LMC); and (3) in the southwestern end the filament orientation gains a significant perpendicular component with respect to the elongation of the SMC main body, which can be tracing gaseous outflows from stellar feedback processes.
Furthermore, we compared the HI filament orientations with starlight polarisation data (Lobo Gomes et al. 2015) that trace the plane-of-sky magnetic field orientations. From this, we find a preferred alignment between these two in SMC volumes where there are reliable starlight polarisation data (mainly to the northeastern end of the SMC main body). This suggests that the HI filamentary structures uncovered by our observations with 9 pc spatial resolution can potentially act as "compass needles" in the SMC, similar to the HI filaments in the solar neighbourhood observed with ~0.1 pc spatial resolution (e.g., McClure-Griffiths et al. 2006; Clark & Hensley 2019). Though, we require comparisons with future plane-of-sky magnetic field measurements that cover the entirety of the SMC to confirm that this is indeed a general characteristic throughout this galaxy.
Earlier this year, the Polarisation Sky Survey of the Universe's Magnetism (POSSUM) survey of ASKAP has also observed the SMC, with the goal of mapping the magnetic field structure (both in the plane-of-sky and along the line-of-sight) within and surrounding this galaxy. We are currently excitedly exploring the synergies between the GASKAP-HI and the POSSUM surveys of ASKAP in furthering our understanding in the magnetism of the SMC, as well as the rest of the Magellanic System and the Milky Way in the next couple of years.
Yik Ki (Jackie) Ma
