Discovery of “fossil” galaxy at the center of the Milky Way, the result of ancient fusion

The red-colored rings represent the extent of the position of the stars that, according to the researchers, once belonged to the Heracles galaxy merged with the Milky Way about ten billion years ago (credit: Danny Horta-Darrington (Liverpool John Moores University) , ESA / Gaia and SDSS)

A sort of “fossil galaxy” was discovered inside the Milky Way thanks to data analysis from the Apache Point Observatory Galactic Evolution Experiment (APOGEE) of the Sloan Digital Sky Surveys. The researchers, who published their study on Monthly Notices of the Royal Astronomical Society, describe Heracles, a sort of “fossil” of a galaxy that should have been taken for granted that then merged with the Milky Way about ten billion years ago, a period in which the Milky Way had just formed.

Name Heracles

The name “Heracles” is not accidental: it is also the name of the god who received immortality as a gift when he was created in the Milky Way in Greek mythology.

One third of the halo of the Milky Way

According to the researchers, the remnants of this galaxy today account for about a third of the spherical halo that surrounds our galaxy. It has not been identified until now, despite this extension, as it is found deep within the galaxy itself. So in depth that the researchers had to analyze in detail the chemical composition and trajectories of tens of thousands of stars, as Ricardo Schiavon, an astronomer at Liverpool John Moores University (LJMU) explains.

Region difficult to observe because it is obscured by dust

This is a very difficult analysis to perform when they concern stars that are in the center of the Milky Way. In fact, in this region there are large and dense clouds of interstellar dust that make observation almost impossible.
The researchers only succeeded with data collected in a large database called APOGEE. The data had been put together between 2011 and 2014 with the technique of high-resolution infrared spectroscopy, which allowed to penetrate the dust that obscures the view of the central area of ​​the galaxy. Thanks to this survey, we were able to learn more about a region that was previously practically unknown to astronomers, a core, that of our galaxy, perpetually obscured by dust.

The position of the stars that researchers believe belonged to the Heracles galaxy in the center of the Milky Way as seen from above, an area heavily obscured by dust (credit: Danny Horta-Darrington (Liverpool John Moores University), NASA / JPL-Caltech and SDSS )

Distinction between “indigenous” stars and “foreign” stars

Then, to distinguish the stars that belonged to the Heracles galaxy from those that were already present in the Milky Way, the researchers created the chemical compositions and speeds of the stars. They realized that a few hundred of them had distinctly different chemical compositions and speeds. So different that the researchers hypothesized that these stars must, inevitably, come from another galaxy.

Major merger event, further analysis will be done

Since the stars that belonged to this galaxy today cover a good area of ​​the Milky Way’s halo, the “collision” between the galaxies must have been quite an important event.
The analysis certainly does not end here: the astronomers now intend to analyze these stars in greater depth to obtain further details of the Heracles galaxy before the merger with the Milky Way and among these details should be its position and how it “collided” with our galaxy.


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