Today it was announced that the halo of the Sombrero galaxy is populated with metal rich stars, that could only have found their way to this weird location through some kind of a merger event in the past. In astronomy terms, metal-rich just means that a star has formed from materials enriched through multiple supernova and other mass loss events. This material has a higher concentration of heavier elements than our universe formed with. In general, metal rich stars can be found most often in the dense disks of galaxies, while the largely empty empty galactic halos are metal poor. In describing this system, the press release uses words like “unexpected,” “defied expectations,” “surprising,” and “unusual.” Now, to be fair, we haven’t seen galaxies with this large a proportion of their halo stars appearing to be metal rich. This is indicative of some sort of a particularly egregious collision sending young metal rich stars in all directions. We know these kinds of events happen because we see them scattered like so many dead bugs all across the sky, and we also know the managed carnage we see during mergers is only a transitory phase – galaxies merge and then they resettle into new larger shapes, include disks like we see with the Sombrero galaxy – this is how you get most massive galaxies – things merge. It is cool that such a beloved object has had such a wild past, and it is remarkable that this team was able to use photometric metallicities of individual stars to get at this result. I love that we now know that galaxies can kinematically settle into such placid looking shapes after experiencing violent mergers.

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Beyond the Brim, Sombrero Galaxy’s Halo Suggests a Turbulent Past  (Hubblesite)