An all-sky image of the stars in the Milky Way as seen from Earth. The coloured rings show the approximate extent of the stars that came from the fossil galaxy known as Heracles. Credit: Danny Horta-Darrington (LJMU), ESA/Gaia, SDSS

Astronomers say they have discovered a fossil galaxy hidden in the depths of the Milky Way that may alter thinking on how our galaxy grew into what we see today.

They have named it Heracles, after the hero of Greco-Roman mythology who received the gift of immortality when the Milky Way was created, and say it may have collided with the Milky Way 10 billion years ago, when our galaxy was still in its infancy.

The remnants of Heracles account for about one-third of the Milky Ways spherical halo, the researchers say in a paper in the Monthly Notices of the Royal Astronomical Society, so this newly discovered ancient collision must have been a major event.

That suggests, they add, that our galaxy may be unusual, since most similar massive spiral galaxies had much calmer early lives.

To find a fossil galaxy like this one, we had to look at the detailed chemical makeup and motions of tens of thousands of stars, says Ricardo Schiavon from Liverpool John Moores University (LJMU) in the UK.

That is especially hard to do for stars in the centre of the Milky Way, because they are hidden from view by clouds of interstellar dust.

To differentiate Heracles from the original Milky Way, the team used chemical compositions and velocities of stars measured by the Sloan Digital Sky Surveys Apache Point Observatory Galactic Evolution Experiment (APOGEE), which takes spectra of stars in near-infrared light.

Of the tens of thousands of stars we looked at, a few hundred had strikingly different chemical compositions and velocities, says LJMUs Danny Horta-Darrington, the papers lead author.

These stars are so different that they could only have come from another galaxy. By studying them in detail, we could trace out the precise location and history of this fossil galaxy.

Because galaxies are built through mergers of smaller galaxies across time, the remnants of older galaxies are often spotted in the outer halo of the Milky Way, a huge but very sparse cloud of stars enveloping the main galaxy.

But since our galaxy built up from the inside out, finding the earliest mergers requires looking at the most central parts of the Milky Ways halo, which are buried deep within the disc and bulge.

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'Fossil galaxy' found deep in the Milky Way - Cosmos