A New Optical Polarization Catalog for the Small Magellanic Cloud: The Magnetic Field Structure

Abstract

We present a new optical polarimetric catalog for the Small Magellanic Cloud (SMC). It contains a total of 7207 stars, located in the Northeast (NE) and Wing sections of the SMC and part of the Magellanic Bridge. This new catalog is a significant improvement compared to previous polarimetric catalogs for the SMC. We used it to study the sky-projected interstellar magnetic field structure of the SMC. Three trends were observed for the ordered magnetic field direction at position angles of $(65\pm 10)$ deg, $(115\pm 10)$ deg, and $(150\pm 10)$ deg. Our results suggest the existence of an ordered magnetic field aligned with the Magellanic Bridge direction and SMC's Bar in the NE region, which have position angles roughly at $115.4$ deg and $45$ deg, respectively. However, the overall magnetic field structure is fairly complex. The trends at $115$ deg and $150$ deg may be correlated with the SMC's bimodal structure, observed in Cepheids' distances and HI velocities. We derived a value of $B_{sky}=(0.947\pm 0.079)\hat{I}¼G$ for the ordered sky-projected magnetic field, and $\hat{I}´B=(1.465\pm 0.069)\hat{I}¼G$ for the turbulent magnetic field. This estimate of $B_{sky}$ is significantly larger (by a factor of $\sim 10$) than the line-of-sight field derived from Faraday rotation observations, suggesting that most of the ordered field component is on the plane of the sky. A turbulent magnetic field stronger than the ordered field agrees with observed estimates for other irregular and spiral galaxies. For the SMC the $B_{sky}/\hat{I}´B$ ratio is closer to what is observed for our Galaxy than other irregular dwarf galaxies.

Updated to match published version (includes extra online figures)