Chemical Abundances in the Leading Arm of the Magellanic Stream

TL;DR

This study measures chemical abundances in the Leading Arm of the Magellanic Stream, revealing low oxygen levels that support a common origin with the Small Magellanic Cloud and suggesting the structure extends further than previously known.

Contribution

First detailed chemical abundance measurements of multiple sightlines in the Leading Arm, providing new insights into its composition and extent.

Findings

Low oxygen abundances (4-13% solar) in the Leading Arm.

Support for a common origin with the Small Magellanic Cloud.

Evidence that the Leading Arm extends at least 20 degrees further northwest.

Abstract

The Leading Arm (LA) of the Magellanic Stream is a vast debris field of H I clouds connecting the Milky Way and the Magellanic Clouds. It represents an example of active gas accretion onto the Galaxy. Previously only one chemical abundance measurement had been made in the LA. Here we present chemical abundance measurements using Hubble Space Telescope/Cosmic Origins Spectrograph Green Bank Telescope spectra of four sightlines passing through the LA, and three nearby sightlines that may trace outer fragments of the LA. We find low oxygen abundances, ranging from 4.0(+4.0,-2.0) percent solar to 12.6(+6.2,-4.1) percent solar, in the confirmed LA directions, with the lowest values found in the region known as LA III, farthest from the LMC. These abundances are substantially lower than the single previous measurement, S/H=35+/-7 percent solar (Lu et al. 1998), but are in agreement with those reported in the SMC filament of the trailing Stream, supporting a common origin in the SMC (not the LMC) for the majority of the LA and the trailing Stream. This provides important constraints for models of the formation of the Magellanic System. Finally, the HVCs in two of the three nearby sightlines show H I columns, kinematics, and oxygen abundances consistent with LA membership. This suggests that the LA is larger than traditionally thought, extending at least 20 degrees further to the Galactic northwest.