“Fossil galaxy” found hidden in the Milky Way

A “fossil galaxy” has been found hidden inside our Milky Way.

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The discovery of the long dead galaxy could change our understanding of the history of the Milky Way and how it has become the vast web of stars surrounding us today.

The fossil, known as Heracles, is thought to have collided with the Milky Way 10 billion years ago, when our galaxy was still very young.

Its remains make up about a third of the spherical halo of the Milky Way, the researchers report. But despite its large scale, astronomers weren’t able to see it until they discovered detailed information on tens of thousands of stars.

Dr Ricardo Schiavon, of Liverpool’s John Moores University Astrophysics Research Institute, said: ‘To’ see ‘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 had to examine 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 had much quieter initial 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 new research is published in The monthly notices of the Royal Astronomical Society.

Additional reporting from Press Association