Thread by @8minutesold

New preprint on the arXiv today to which I had the honor to contribute a little: "The Magellanic System: the puzzle of the leading gas stream", lead by Thor Tepper-García and @JossBlandHawtho. arxiv.org/abs/1901.05636

Let me try to summarize it in a short thread.

The Magellanic Stream is a gas structure emanating from the Magellanic Clouds that stretches over 150º across the south of the Milky Way, behind the Clouds. In front there's a gaseous feature called the Leading Arm - usually thought to be the Stream's counterpart running ahead.

Many properties of the Stream, as well as a Leading Arm-like feature, can be reproduced in simulations that model a mutual tidal interaction of the Magellanic Clouds during their first-time infall into the Milky Way halo.

See e.g. Besla et al. (2012, arxiv.org/abs/1201.1299), the source of the previous plot, or Pardy et al. (2018, arxiv.org/abs/1802.01600), on whose model the simulations in the new paper are based.

These simulations have thus far neglected the well established presence of a hot corona around the Milky Way. Such a diffuse, hot, gaseous component around the Galaxy affects the evolution and survival of gaseous features in the halo.

Using hydrodynamical simulations that include a hot corona (including weakly magnetized and spinning coronas), Thor now shows that no Leading Arm feature forms, due to the hydrodynamical interaction of the MCs' gas and the Milky Way's corona. (Plot 1: no corona, 2: with corona).

Well, that's a problem of course. It leaves two possibilities:

1) Either, the Leading Arm survives because the MW corona is less dense, by a factor of ~10, than usually assumed. This could be consistent with a rapidly rotating corona, but would not leave enough baryons in the hot halo to solve the "missing baryons" problem.

2) Alternatively, the `Leading Arm' might have nothing to do with the Magellanic Clouds. Instead, it could be caused by a "frontrunner", a satellite galaxy moving ahead of the Clouds that got its gas stripped.

We checked using our proper motion measurements and there are indeed such satellites ahead of the Clouds moving in the right direction. None of them match perfectly though, e.g. no close passage to MW or no recent star formation as expected if they had gas until recently.

What's interesting though is that the 2nd possibility resembles a scenario proposed by Hammer et al. (2015, arxiv.org/abs/1510.00096), who argued that the Clouds as well as other, front-running satellites are dark matter free Tidal Dwarf Galaxies suffering ram-pressure stripping.

However, since neither of the two possibilities is without problems, the main conclusion I take away from this is: we are facing a real puzzle here regarding the origin of the Leading Arm.