Hα Distances to the Leading Arm of the Magellanic Stream

Abstract

The Leading Arm (LA) is a tidal feature that is in front of the Magellanic Clouds (MCs) on their orbit through the Galaxy’s halo. Many physical properties of the LA, such as its mass and size, are poorly constrained because it has few distance measurements. While H α measurements have been used to estimate the distances to halo clouds, many studies have been unsuccessful in detecting H α from the LA. In this study, we explore a group of H i clouds which lie 75°–90° from the MCs. Through ultraviolet and 21 cm radio spectroscopy, this region, dubbed the LA Extension, was found to have chemical and kinematic similarities to the LA. Using the Wisconsin H α Mapper, we detect H α emission in four out of seven of our targets. Assuming that this region is predominantly photoionized, we use a radiation model that incorporates the contributions of the Galaxy, MCs, and the extragalactic background at z  = 0 to derive a heliocentric distance of d ⊙  ≥ 13.4 . We also use this model to rederive H α distances of d ⊙  ≥ 5.0 kpc and d ⊙  ≥ 22.9 to two clouds in the literature that might also be associated with the LA. Using these new measurements, and others in the literature, we provide a general trend of the variation of LA heliocentric distance as a function of Magellanic Stream longitude, and explore its implications for the origin and closest point of approach of the LA.