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Astronomers have discovered a “fossil galaxy” hidden deep in the Milky Way.
Dubbed Heracles, the dead galaxy remained from the early universe and is thought to have collided with the Milky Way about 10 billion years ago.
Researchers say the remains of Heracles account for about one third of the spherical “halo” of the Milky Way, which is made up of star clusters, gas and dust.
Scientists have said that the remnants of older galaxies can often be found in the outer halo of the Milky Way, because the galaxies are built through mergers of smaller galaxies.
However, they added, the first mergers require to look at the more central parts of the halo of the Milky Way, buried deep within the disc and the bulge.
Dr. Ricardo Schiavon, the institute of astrophysics research of John Moores University of Liverpool, said: ” ‘To perceive’ that galaxy is fantastic.
“It’s really small in the cosmological context – only 100 million stars – but it accounts for nearly half the mass of the entire Milky Way halo.”
A team of astronomers led by Dr. Schiavon analyzed data from the Apache Point Observatory Galactic Evolution Experiment (Apogee) project, which has amassed vast amounts of information on more than half a million stars across the Milky Way.
Dr. Schiavon said: “To find a fossil galaxy like this, we have examined the detailed chemical composition and movements of tens of thousands of stars.
“This is particularly 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.”
To separate the stars belonging to Heracles from those of the original Milky Way, the team used Apogee instruments to measure the chemical compositions and speeds of the stars.
Danny Horta, a graduate student at Liverpool John Moores University, said: “Of the tens of thousands of stars we observed, a few hundred had surprisingly different chemical compositions and speeds.
“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.”
Based on their findings, the researchers say the collision between Heracles and the Milky Way “must have been an important event in the history of our galaxy.”
They believe this makes the Milky Way unusual because “the most similar massive spiral galaxies have had much quieter lives.”
Dr Schiavon said: “As our cosmic home, the Milky Way is already special to us, but this ancient galaxy buried within it makes it even more special.”
The findings are published in The Monthly Notice of The Royal Astronomical Society.
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