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.
It is believed that the fossil, known as Heracles, 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, researchers report. But despite its large scale, astronomers were unable to see it until they revealed detailed information about tens of thousands of stars.
Dr Ricardo Schiavon, of Liverpool John Moore̵
“It’s really small in the cosmological context – only 100 million stars – but accounts for almost half the mass of the entire Milky Way halo.”
A team of astronomers led by Dr. Schiavon analyzed the data from the Apache Point Observatory Galactic Evolution Experiment (Apogee) project, which has gathered large amounts of information about more than half a million stars across the Milky Way.
Dr Schiavon said: “To find a fossil galaxy like this, we had to look at the detailed chemical composition and motions of tens of thousands of stars.
“It is especially difficult to do for stars in the middle of the Milky Way because they are hidden from view by clouds of interstellar dust.
“Apogee lets us penetrate that dust and look deeper into the heart of the Milky Way than ever before.”
To distinguish stars belonging to Heracles from those of the original Milky Way, the team used Apogee instruments to measure the chemical compositions as well as the speed of the stars.
Danny Horta, a graduate student at Liverpool’s John Moore’s University, said: ‘Of the tens of thousands of stars we looked at, a few hundred had markedly different chemical compositions and velocities.
“These stars are so different that they could only have come from another galaxy.
“By studying them in detail, we were able to trace the exact location and history of this fossil galaxy.”
Based on their findings, the scientists say that the collision between Heracles and the Milky Way “must have been an important event in our galaxy history”.
They believe this makes the Milky Way unusual because “most similar massive spiral galaxies had much calmer early lives”.
Dr Schiavon said, “Like our cosmic home, the Milky Way is already special to us, but this ancient galaxy buried inside makes it even more special.”
The new research is published in The monthly announcements from the Royal Astronomical Society.
Additional reporting from the Press Association