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The discovery of a “fossil galaxy” hidden deep within our galaxy, the Milky Way, could alter our understanding of how it became the galaxy we see today.
The discovery of the Sloan Digital Sky Surveys’ Apache Point Observatory (Apogee) galactic evolution experiment was published in the Royal Astronomical Society’s monthly notices.
This fossil galaxy may have hit the Milky Way 10 billion years ago, when our galaxy was still in its infancy. Astronomers named it Heracles, named after the ancient Greek hero who received the gift of immortality during the creation of the Milky Way.
The remains of Heracles make up about one third of the spherical halo of the Milky Way. But if the stars and the gas of Heracles make up such a high percentage of the galactic halo, why haven’t we seen it before? The answer lies in its location in the heart of the Milky Way.
‘To find a fossil galaxy like this, we had to examine the detailed chemical composition and motions of tens of thousands of stars,’ says Ricardo Schiavon of John Moores University in Liverpool (LJMU) in the UK, a key member of the research team. “This is particularly difficult for the stars at 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 the course of its ten years of observational life, Apogee has measured the spectra of more than half a million stars across the Milky Way, including its previously obscured core.
Like needles in a haystack
LJMU graduate student Danny Horta, lead author of the paper reporting the result, explains that “you have to scan 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 the stars belonging to Heracles from those of the original Milky Way, the team used both the chemical compositions and the velocities of the stars measured by the Apogee instrument.
“Of the tens of thousands of stars we observed, a few hundred had significantly 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 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, to find the first mergers it is necessary to examine the most central parts of the Milky Way halo, which are buried deep in 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 recently discovered ancient collision must have been a major event in history. of our galaxy. This suggests our galaxy may be unusual, as most similar massive spiral galaxies had much quieter initial lives.
“Just 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 extraordinary 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 Apogee’s observations. The fifth phase of SDSS has already begun to collect data and its “Milky Way Mapper” will build on Apogee’s success in measuring spectra ten times more stars in all parts of the Milky Way. , using near infrared visible light. and sometimes both.
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