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 (PAs) of (65◦ ± 10◦), (115◦ ± 10◦), and (150◦ ± 10◦). 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 PAs roughly at 115◦.4 and 45◦, respectively. However, the overall magnetic field structure is fairly complex. The trends at 115◦ and 150◦ may be correlated with the SMC’s bimodal structure, observed in Cepheids’ distances and HI velocities. We derived a value of Bsky = (0.947 ± 0.079) µG for the ordered sky-projected magnetic field, and δB = (1.465±0.069) µG for the turbulent magnetic field. This estimate of Bsky is significantly larger (by a factor of ∼ 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 Bsky/δB ratio is closer to what is observed for our Galaxy than other irregular dwarf galaxies.