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A “fossil galaxy” has been found hidden inside our Milky Way.
The discovery of the long-dead galaxy could change our understanding of the history of the Milky Way and how it became the vast network of stars that surround us today.
The fossil, known as Heracles, is believed to have collided with the Milky Way 10 billion years ago, when our galaxy was still at a very young age.
Its remains make up about a third of the Milky Way’s spherical halo, the researchers report. But despite its large scale, astronomers are unable to see it until they have discovered detailed information on tens of thousands of stars.
Dr Ricardo Schiavon, from the Institute for Research in Astrophysics, John Moores University in Liverpool, said: “To pin down this galaxy is awesome.
“It’s really small in the cosmological context – just 100 million stars – but makes up almost 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), which amassed vast amounts of information on more than half a million stars across the Way. milky.
Dr Schiavon said: “To find a fossil galaxy like this, we had to examine the detailed chemical makeup and motions of tens of thousands of stars.
“This is especially difficult to do for the stars in the center of the Milky Way, because they are hidden by clouds of interstellar dust.
“Apogee allows us to pierce through the dust and see deeper into the heart of the Milky Way than ever before. “
To separate stars belonging to Heracles from those in the original Milky Way, the team used Apogee instruments to measure chemical compositions as well as star velocities.
Danny Horta, a graduate student at John Moores University of Liverpool, said: “Of the tens of thousands of stars we reviewed a few hundred had chemical compositions and extremely different speeds.
“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.”
Based on their findings, the researchers claim that the collision between Heracles and the Milky Way “must have been a major event in the history of our galaxy.”
They believe this makes the Milky Way unusual because “most similar massive spiral galaxies had much quieter lives early on.”
Dr. Schiavon said: “As the cosmic home, the Milky Way is already special for us, but this old galaxy buried inside makes it even more special.”
The new research is published in Monthly notices from the Royal Astronomical Society.
Additional reports from the Press Association
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