Astronomers discover a new “fossil galaxy” buried deep in the Milky Way

Scientists working with data from the Sloan Digital Sky Surveys’ Apache Point Observatory Galactic Evolution Experiment (APOGEE) have discovered a “fossil galaxy” hidden deep within our Milky Way.

This result, published today in Royal Astronomical Society Monthly Notices, could shake our understanding of how the Milky Way grew into the galaxy we see today.

The proposed fossil galaxy may have collided with the Milky Way ten billion years ago, when our galaxy was still in its infancy. Astronomers named 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 about a third of the spherical halo of the Milky Way. But if the stars and Heracles’ gas make up such a high percentage of the galactic halo, why haven’t we seen it before? The answer lies in its location deep in the Milky Way.

“To find a fossil galaxy like this, we had to look at the detailed chemical composition and 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. “This is especially difficult for stars at 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.”

APOGEE does this by taking spectra of stars in near infrared light, instead of visible light, which is obscured by dust. Over the course of its ten-year observation life, APOGEE measured the spectra of more than half a million stars across the Milky Way, including its previously obscured core.

LJMU PhD 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 the Milky Way. a haystack. “

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

“Of the tens of thousands of stars we looked at, a few hundred had strikingly different chemical compositions and speeds,” said Horta. “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.”

Because galaxies are constructed through mergers of smaller galaxies over time, the remnants of older galaxies are often spotted in the outer halo of the Milky Way, a huge but very sparse cloud of stars that envelops the main galaxy. But because our galaxy formed from the inside out, to find the first mergers you need to look at the most central parts of the Milky Way halo, which are buried deep in the disk and bulge.

Stars originally belonging to Heracles now account for about a third of the mass of the entire Milky Way halo, meaning this recently discovered ancient collision must have been a major event in the history of our galaxy. This suggests our galaxy may be unusual, since most similar massive spiral galaxies have had much quieter initial lives.

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

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

Provided by Sloan Digital Sky Survey