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Scientists named it Heracles, in honor of the ancient Greek hero who received the gift of immortality when the Milky Way was created.
Scientists working with data from the Sloan Digital Sky Surveys Apache Point Observatory (Apogee) Galactic Evolution Experiment have discovered a “fossil galaxy” hidden deep within our Milky Way.
This result, published this Friday 20-N in the “Monthly Notices of the Royal Astronomical Society,” could alter our understanding of how the Milky Way became the galaxy we see today.
The proposed fossil galaxy may have collided with the Milky Way 10 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 make up about one 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 examine the detailed chemical composition and motions of tens of thousands of stars,” says Ricardo Schiavon of Liverpool John Moores University (LJMU) in the UK, a key member of the team. Research.
“This is particularly difficult for stars in the center of the Milky Way to do, 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 observational life, Apogee measured the spectra of more than half a million stars across the Milky Way, including its previously obscured core.
LJMU graduate student Danny Horta, lead author of the paper announcing the result, explains that “you have to scan so many 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 and the speeds of the stars measured by the APOGEE instrument.
“Of the tens of thousands of stars we see, a few hundred had surprisingly 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, the 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 because our galaxy was built from the inside out, to find the first mergers it is necessary to look at the more central parts of the Milky Way halo, which are buried deep in the disk and protrude.
The stars that originally belonged to Heracles now 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 our galaxy’s history. This suggests our galaxy may be unusual, as most similar massive spiral galaxies 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, 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 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 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 of ten times more stars in all parts of the Milky Way, using visible near-infrared light. and sometimes both.
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