H$\alpha$ Distances to the Leading Arm of the Magellanic Stream

Abstract

The Leading Arm is a tidal feature that is in front of the Magellanic Clouds on their orbit through the Galaxy's halo. Many physical properties of the Leading Arm, such as its mass and size, are poorly constrained because it has few distance measurements. While H$\alpha$ measurements have been used to estimate the distances to halo clouds, many studies have been unsuccessful in detecting H$\alpha$ from the Leading Arm. In this study, we explore a group of H I clouds which lie $75^{\circ }-90^{\circ }$ from the Magellanic Clouds. Through ultraviolet and 21-cm radio spectroscopy, this region, dubbed the Leading Arm Extension, was found to have chemical and kinematic similarities to the Leading Arm. Using the Wisconsin H$\alpha$ Mapper, we detect H$\alpha$ 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, Magellanic Clouds, and the extragalactic background at $\mathrm{z}=0$ to derive a heliocentric distance of $d_{\odot }\ge 13.4kpc$. We also use this model to rederive H$\alpha$ distances of $d_{\odot }\ge 5.0$ kpc and $d_{\odot }\ge 22.9kpc$ to two clouds in the literature that might also be associated with the Leading Arm. Using these new measurements, and others in the literature, we provide a general trend of the variation of Leading Arm heliocentric distance as a function of Magellanic Stream longitude, and explore its implications for the origin and closest point of approach of the Leading Arm.

arXiv comment13 pages, 5 figures, published in ApJ

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• Mainly on the Plane: Observing the Extended, Ionized Disks of Milky Way Analogs in IllustrisTNG