The Magellanic Stream: break up and accretion onto the hot Galactic corona (1505.01587v2)

Abstract: The Magellanic HI Stream (~2x10^9 Msun [d/55 kpc]^2) encircling the Galaxy at a distance 'd' is arguably the most important tracer of what happens to gas accreting onto a disk galaxy. Recent observations reveal that the Stream's mass is in fact dominated (3:1) by its ionised component. Here we revisit the origin of the mysterious H-alpha recombination emission observed along much of its length that is overly bright (~150-200 milli-Rayleigh) for the known Galactic ultraviolet background (~20-40 mR / [d/55 kpc]^2). In an earlier model, we proposed that a slow shock cascade was operating along the Stream due to its interaction with the extended Galactic hot corona. We find that, for a smooth coronal density profile, this model can explain the bright H-alpha emission if the coronal density satisfies 2 < (n / 10^{-4} cm^{-3}) < 4 at d = 55 kpc. But in view of updated parameters for the Galactic halo and mounting evidence that most of the Stream must lie far beyond the Magellanic Clouds (d>55 kpc), we revisit the shock cascade model in detail. At lower densities, the HI gas is broken down by the shock cascade but mostly mixes with the hot corona without significant recombination. At higher densities, the hot coronal mass (including the other baryonic components) exceeds the baryon budget of the Galaxy. If the H-alpha emission arises from the shock cascade, the upper limit on the smooth coronal density constrains the Stream's mean distance to < 75 kpc. If, as some models indicate, the Stream is even further out, either the shock cascade is operating in a regime where the corona is substantially mass-loaded with recent gas debris, or an entirely different ionization mechanism is responsible.