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A new study could revolutionize our understanding of how the Milky Way grew into the galaxy we see today. It suggests that a “fossil galaxy” is hidden deep within our Milky Way.
The results come from data observed from the Apache Point Observatory of the Sloan Digital Sky Surveys Galactic Evolution Experiment (APOGEE).
According to astronomers, this fossil galaxy may have collided with the Milky Way 10 billion years ago. Astronomers dubbed this fossil galaxy as Heracles after the ancient Greek hero received the gift of immortality when the Milky Way was created.
The remains of Heracles represent about a third of the spherical halo of the Milky Way. However, on the off chance that the stars and Heracles gas make up such a high percentage of the galactic halo, why did we miss it? The answer lies in its location somewhere within the Milky Way.
Ricardo Schiavon of Liverpool John Moores University (LJMU) in the UK, a vital member of the research team, said: “To find a fossil galaxy like this, we had to examine the detailed chemical composition and motions of tens of thousands of stars. This is particularly difficult for stars in the center of the Milky Way to do because they are hidden from view by clouds of interstellar dust. APOGEE allows us to penetrate through that dust and see deeper into the heart of the Milky Way than ever. “
LJMU graduate student Danny Horta, the lead author of the paper announcing the result, explains: “Examining such a large number of stars is necessary to find unusual stars in the densely populated heart of the Milky Way, which is like finding needles in a haystack.”
The team used the chemical compositions and speeds of stars measured by the APOGEE instrument to separate the stars belonging to Heracles from those of the original Milky Way.
Horta said, “Of the tens of thousands of stars we observed, a few hundred had surprisingly different chemical compositions and speeds. These stars are so different that they could only come from another galaxy. By studying them in detail, we could trace the exact location and history of this fossil galaxy. “
The stars belonging to this fossil galaxy account for about one third of the entire mass of the Milky Way’s halo. It means that this recently discovered ancient collision must have been an important event in the history of our galaxy.
Schiavon said, “As our cosmic home, the Milky Way is already special to us, but this ancient galaxy buried within it makes it even more special.”
Karen Masters, the spokesman of SDSS-IV, comments, “APOGEE is one of the flagship surveys of the fourth phase of SDSS, and this result is an example of the extraordinary science that anyone can do now that we have nearly completed our 10-year mission.”
Journal reference:
- Evidence from APOGEE for the Presence of a Major Building Block of the Halo Buried in the Inner Galaxy “, Danny Horta et al., 20 November 2020, Royal Astronomical Society Monthly Notices: arxiv.org/abs/2007.10374
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