The Origin of the Magellanic Stream and its Leading Arm

Abstract

We explore the Magellanic Stream (MS) using a Gaussian decomposition of the H I velocity proles in the Leiden-Argentine-Bonn (LAB) all-sky H I survey. This decomposition exposes the MS to be composed of two laments distinct both spatially (as rst pointed out by Putman et al.) and in velocity. Using the velocity coherence of the laments, one can be traced back to its origin in what we identify as the SouthEast H I Overdensity (SEHO) of the Large Magellanic Cloud (LMC), which includes 30 Doradus. Parts of the Leading Arm (LA) can also be traced back to the SEHO in velocity and position. Therefore, at least one-half of the trailing Stream and most of the LA originates in the LMC, contrary to previous assertions that both the MS and the LA originate in the Small Magellanic Cloud (SMC) and/or in the Magellanic Bridge. The two MS laments show strong periodic, undulating spatial and velocity patterns that we speculate are an imprint of the LMC rotation curve. If true, then the drift rate of the Stream gas away from the Magellanic Clouds is 49 km s 1 and the age of the MS is 1.74 Gyr. The Staveley-Smith et al. high-resolution H I data of the LMC show gas outows from supergiant shells in the SEHO that seem to be creating the LA and LMC lament of the MS. Blowout of LMC gas is an eect not previously accounted for but one that probably plays an important role in creating the MS and LA. Subject headings: Galaxies: interactions { Galaxies: kinematics and dynamics { Galaxies: Local Group { Galaxy: halo { Intergalactic Medium { Magellanic Clouds { Radio Lines: general