The discovery of a “fossil galaxy“Hidden deep within our own Milky Way, it can alter our understanding of how the galaxy we see today became.

The finding from the Sloan Digital Sky Surveys Apache Point Observatory Galactic Evolution Experiment (APOGEE) has been published in Monthly Notices of the Royal Astronomical Society.

This fossil galaxy may have collided with the Milky Way 10,000 million years ago, when our galaxy was still in its childhood. Astronomers have called it Heracles, in honor of the ancient Greek hero who received the gift of immortality when the Milky Way was created.

The remains of Heracles represent approximately one third of the spherical halo of the Milky Way. But if Heracles’s stars and gas make up such a large percentage of the galactic halo, why didn’t we see it earlier? The answer is in your Location in the depths of the Milky Way.

“To find a fossil galaxy like this one, we had to look at the detailed chemical composition and the movements of tens of thousands of stars, “says Ricardo Schiavon of Liverpool John Moores University (LJMU) in the UK, a key member of the research team.” That is especially difficult for stars to do. in the center of the Milky Way, because they are hidden from view by clouds of interstellar dust. APOGEE allows us to break through that dust and see deeper than ever into the heart of the Milky Way. “

APOGEE does this by taking spectra of stars in near-infrared light, rather than visible light, which is obscured by dust. During its 10-year observing life, APOGEE has measured the spectra of more than half a million stars throughout the Milky Way, including its previously dust-obscured core.

Like needles in a haystack

LJMU graduate student Danny Horta, lead author of the article announcing the result, explains that “you have to examine such a large number of stars to find unusual stars in the densely populated heart of the Milky Way, which is like finding needles in a haystack “.

To separate the stars belonging to Heracles from those of the original Milky Way, the team used both the chemical compositions like the star speeds measured by the APOGEE instrument.

“Of the tens of thousands of stars we observed, a few hundred had strikingly different chemical compositions and speeds,” Horta said. “These stars are so different that they could only have come from another galaxy. By studying them in detail, we could trace the precise location and history of this fossil galaxy. “

They discover a planet the size of Earth wandering the Milky Way and without a parent star

Because galaxies are built through mergers of smaller galaxies over time, remnants of older galaxies are often seen in the outer halo of the Milky Way, a huge but very sparse cloud of stars that envelops the main galaxy. But since our galaxy was built of inside out, find the first mergers requires looking at the most central parts of the Milky Way halo, which are buried deep inside the disc and bulge out.

The stars that originally belonged to Heracles make up about a third of the mass of the entire Milky Way halo today, meaning this newly discovered ancient collision must have been a Important event in the history of our galaxy. That suggests that our galaxy may be unusual, since most similar massive spiral galaxies had much calmer early lives.

“Like our cosmic home, the Milky Way is already special to us, but this ancient galaxy buried within it makes it even more special,” says Schiavon.

Karen Masters, spokesperson for SDSS-IV comments: “APOGEE is one of the flagship surveys of the fourth phase of SDSS, and this result is an example of the amazing science that anyone can do, now that we have almost completed our ten year mission. “.

And this new era of discovery will not end with the completion of observations. APOGEE. The fifth phase of the SDSS has already begun to collect data, and its ‘Milky Way Mapper’ will build on the success of APOGEE in measuring spectra of ten times more stars in all parts of the Milky Way, using near infrared, visible light. and sometimes both.