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A fossil galaxy lurked deep in the Milky Way

The discovery of a “fossil galaxy” hidden deep within our own Milky Way galaxy may alter our understanding of how it became the galaxy we see today.

The discovery of Sloan Digital Sky Surveys’ Apache Point Observatory (Apogee) galactic evolution experiment was published in the Monthly Notices of the Royal Astronomical Society.

This fossil galaxy may have struck the Milky Way 10 billion years ago, when our galaxy was still in its infancy. Astronomers named him Heracles, after the name of the ancient Greek hero who received the gift of immortality during the creation of the Milky Way.

Heracles’ remains make up about a third of the spherical halo of the Milky Way. But if Heracles’ stars and gas make up such a large percentage of the galactic halo, why haven’t we seen it sooner? The answer lies in its location in the heart of the Milky Way.

“To find a fossil galaxy like this, we had to look at the detailed chemical makeup and motions of tens of thousands of stars,” says Ricardo Schiavon of John Moores University in Liverpool (LJMU) in the UK, key member of the research team. “This is especially difficult for the stars in the center of the Milky Way, as they are hidden from view by clouds of interstellar dust. Apogee allows us to pierce 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. Over its ten-year observational life, Apogee has measured the spectra of more than half a million stars throughout the Milky Way, including its previously obscured core with dust.

Like needles in a haystack

LJMU graduate student Danny Horta, lead author of the article reporting the result, explains that “you have to examine so many stars to find unusual stars in the densely populated core of the Milky Way, which is like finding needles in a haystack “.

To separate stars belonging to Heracles from those in the original Milky Way, the team used both the chemical compositions and the star velocities measured by the Apogee instrument.

“Of the tens of thousands of stars that we observed, a few hundred had remarkably different chemical compositions and velocities,” 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.”

Because galaxies are built through mergers of smaller galaxies over time, remnants of older galaxies are often seen in the Milky Way’s Outer Halo, a huge but very sparse cloud of stars that envelops the galaxy. main. But because our galaxy was built from the inside out, finding the first mergers requires examining the more central parts of the Milky Way’s halo, which are buried deep within the disk and swell.

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

“Much 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 investigations 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 the Apogee observations. The fifth phase of SDSS has already started collecting data, and its “ Milky Way Mapper ” will build on Apogee’s success in measuring spectra ten times as many stars in all parts of the Milky Way. , using visible light in the near infrared. and sometimes both.

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