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H3 paper day! Here we find something super intriguing about the iconic Sagittarius stream whose debris spectacularly garlands the Milky Way. (sidenote: Sgr was discovered 3 weeks after I was born, maybe I was fated for Galactic archaeology all along!?)
https://arxiv.org/abs/2007.14408
https://arxiv.org/abs/2007.14408
About 5+ billion years ago the Sagittarius dwarf galaxy entered the Milky Way's sphere of influence. Our home Galaxy welcomed it with shredding tidal forces that spread Sagittarius' stars out in spectacular, ribbon-like streams that criss-cross the Galaxy.
While the ribbon-like, kinematically cold features of the Sagittarius debris (the "trailing arm" and "leading arm") have been studied for decades now, in this paper we discover a hot, metal-poor, diffuse population! (the smattering of stars off the main-stream tracks)
We ran a lot of checks: these metal-poor stars that lie "off the stream" are well-aligned with Sagittarius in their proper motions, angular momenta, energy. In all ways, but for their kinematic hotness, they trace the orbit of the stream!
Models indicate we are likely seeing the *halo* of Sgr--its outer-most, diffuse, metal-poor reaches, which were shed as it first entered our Galaxy. The halos of dwarf galaxies are quite challenging to study, so its pretty neat that we now have individual stars from one of them!
I really like this paper because it finds something super new about perhaps the most-studied object in the Milky Way. It's a nice antidote to the feeling of "ahh everything worth doing has already been done so why are we even trying".
I'll end this thread by noting that Ben Johnson, who led this paper, is one of the kindest folks I know in Astronomy! I have been lucky to partake his wisdom on things ranging from SEDs of the first galaxies to the nitty gritty of stellar spectra--a true Renaissance astronomer!
