Probing the Structure and Kinematics of the transition Layer between the Magellanic Stream and the Halo in HI

Abstract

The Magellanic Stream (MS) is a nearby laboratory for studying the fate of cool gas streams injected into a gaseous galactic halo. We investigate properties of the boundary layer between the cool MS gas and the hot Milky Way halo with 21 cm HI observations of a relatively isolated cloud having circular projection in the northern MS. Through averaging and modeling techniques, our observations obtained with the Robert C. Byrd Green Bank Telescope (GBT), reach unprecedented 3σ sensitivity of ~10^17/cm^2, while retaining the telescope's 9.1' resolution in the essential radial dimension. We find an envelope of diffuse neutral gas with FWHM of 60 km/s, associated in velocity with the cloud core having FWHM of 20 km/s, extending to 3.5 times the core radius with a neutral mass seven times that of the core. We show that the envelope is too extended to represent a conduction-dominated layer between the core and the halo. Its observed properties are better explained by a turbulent mixing layer driven by hydrodynamic instabilities. The fortuitous alignment of the NGC 7469 background source near the cloud center allows us to combine UV absorption and HI emission data to determine a core temperature of 8350 +/- 50 K. We show that the HI column density and size of the core can be reproduced when a slightly larger cloud is exposed to Galactic and extragalactic background ionizing radiation. Cooling in the large diffuse turbulent mixing layer envelope extends the cloud lifetime by at least a factor of two relative to a simple hydrodynamic ablation case, suggesting that the cloud is likely to reach the Milky Way disk.

Accepted for publication in ApJ. 19 pages, 9 figures, 3 tables

References in corpus (15)

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• Fossil imprint of a powerful flare at the Galactic Centre along the Magellanic Stream
• The COS/UVES Absorption Survey of the Magellanic Stream: I. One-Tenth Solar Abundances along the Body of the Stream
• The large-scale ionization cones in the Galaxy
• Large scale distribution of arrival directions of cosmic rays detected above $10^{18}$ eV at the Pierre Auger Observatory
• Revealing the Ionization Properties of the Magellanic Stream using Optical Emission
• Radiative Turbulent Mixing Layers and the Survival of Magellanic Debris
• Cosmic Rays around $10^{18}$eV: Implications of Contemporary Measurements on the Origin of the Ankle Feature
• Tracing dense and diffuse neutral hydrogen in the halo of the Milky Way