Magnetised HI superbubbles in the Small Magellanic Cloud revealed by the POSSUM pilot survey

TL;DR

This study detects and analyzes magnetic fields in HI superbubbles within the Small Magellanic Cloud using the POSSUM survey, revealing enhanced magnetic fields at shell edges and demonstrating the effectiveness of dense RM grids in extragalactic magnetic studies.

Contribution

First detection of magnetic fields associated with HI superbubbles in the SMC using high-density RM measurements from the POSSUM survey, supported by MHD simulations.

Findings

Magnetic fields at shell edges are ~1 μG stronger than at the center.

Magnetic field strength in shells is an order of magnitude larger than ambient medium.

RM profiles match predictions from magneto-hydrodynamic models.

Abstract

Neutral hydrogen (HI) bubbles and shells are common in the interstellar medium (ISM). Studying their properties provides insight into the characteristics of the local ISM as well as the galaxy in which the bubbles reside. We report the detection of magnetic fields associated with superbubbles in the nearby irregular galaxy, the Small Magellanic Cloud (SMC). Using the Polarisation Sky Survey of the Universe's Magnetism (POSSUM) pilot survey, we obtain a high-density grid ($\approx 25 \,\rm sources\,deg^{-2}$) of Faraday rotation measure (RM) from polarized sources behind the SMC. This provides a sufficiently large number of RM measurements to study the magnetic properties of three of the largest HI shells previously identified in the SMC. The RM profiles as a function of distance from the shell centre show characteristic patterns at angular scales comparable to the shell size. We demonstrate that this can be explained by magneto-hydrodynamic simulation models of bubbles expanding in magnetised environments. From the observations, we estimate the line-of-sight magnetic field strength at the edges of the shells is enhanced by $\sim1\,\rm \mu G$ with respect to their centres. This is an order of magnitude larger than the field strength in the ambient medium ($\sim 0.1\,\rm \mu G$) estimated based on the expansion velocity of the shells. This paper highlights the power of densely mapped RM grids in studying the magnetic properties of galactic substructures beyond the Milky Way.