A remarkable amount of astronomy can be summed up as “we looked in the direction we’d looked in a million times, but we used this new thing – this new instrument, technique, spacecraft – and we discovered that something old and familiar is very different from what we thought.”

Last week we brought you a story on how studies of globular clusters had shown how our Milky Way galaxy has been built up through merger after merger with other galaxies. We also showed how a population of very metal-poor stars in our galaxy’s disk indicated that our entire system had been affected by these mergers. Well, as often seems to happen in astronomy and new TV seasons, we have another story with essentially the same plot line. 

The APOGEE spectrograph, which the Sloan Digital Sky Survey (SDSS) used to measure the metalicities of more than half a million stars, has determined there is a blob of stars down in the heart of our galaxy that is all made of roughly the same material and shares related orbits. This seems to indicate that these stars come from a single, massive population that crashed into our system. When I say massive, I mean this population makes up fully one-third of the stars in our galaxy’s spherical halo.

This work is described in a new paper in Monthly Notices of the Royal Astronomical Society (MNRAS) with lead author Danny Horta.

According to team member Ricardo Schiavon: To find a fossil galaxy like this one, we had to look at the detailed chemical makeup and motions of tens of thousands of stars. That is especially hard to do for stars in the center of the Milky Way, because they are hidden from view by clouds of interstellar dust. APOGEE lets us pierce through that dust and see deeper into the heart of the Milky Way than ever before.”

As we develop more and more telescopes working in these longer, dust penetrating wavelengths, and as our data on stellar motions improves, we’re probably going to discover more and more of these chemically and kinematically linked star populations. This will essentially allow us to unmix our galaxy’s stars and see just what we’re made of, and I can’t wait to learn more about its remarkable mixing pot history.

More Information

SDSS press release 

“Evidence from APOGEE for the Presence of a Major Building Block of the Halo Buried in the Inner Galaxy,” Danny Horta et al., 2020 Nov. 20, Monthly Notices of the Royal Astronomical Society (preprint on arxiv.org)