An all-sky image of the stars in the Milky Way as seen from Earth. The colored rings show the approximate extent of the stars that came from the fossil galaxy known as Heracles. The small objects to the lower right of the image are the Large and Small Magellanic Clouds, two small satellite galaxies of the Milky Way.(Danny Horta-Darrington

While astronomers keep discovering one peculiar galaxy after another every other day, the latest study shows that we missed one ancient galaxy, much closer to our home. Scientists have now discovered a fossil galaxy hidden deep inside our Milky Way galaxy. This newly discovered ancient galaxy has been named after the ancient hero Heracles, who, according to Greek mythology, received the gift of immortality when the Milky Way was created.

Scientists say that the results of this discovery can alter our understanding of how the Milky Way evolved as a galaxy and could explain some of its peculiar properties. However, the location of this new mystery galaxy was buried within our galaxy in such a way that the astronomers failed to spot it for so long despite the Milky Way being the most studied galaxy yet.

Larger galaxies evolve by merging smaller galaxies over time. The remnants of the older galaxies are often spotted on the outer haloa large cloud of scattered stars enveloping the main galaxy. The build of the Milky Way is inside out and hence to find the earliest merger of galaxies scientists look at the central part of the halo, which is deep within the disc and the bulge.

As per the study, the fossil galaxy is said to have collided with the Milky Way ten billion years agoa time when the galaxies were still in their infant stages. This might have been an important event in the history of the Milky Way as stars that were originally from Heracles makeup around one-third of the mass of the entire Milky Way halo today. This also points out that the Milky way is probably much different than the other galaxies, as most of them are known to have a less-chaotic early life.

The data from the Sloan Digital Sky Surveys' Apache Point Observatory Galactic Evolution Experiment (APOGEE) was used to discover the fossil galaxy.

For the research, the scientists took spectra of stars in near-infrared light, as visible light gets obscured by dust. Observations were made for ten years and more than half a million stars across the Milky way were measured, including those in the core that was obscured by dust previously.

Examining such large numbers of stars is important to identify unusual stars in the centre of the Milky Way. To categorise stars into that of Milky Way and Heracles, the team measured chemical compositions and velocities of stars using the APOGEE instrument.

An artists impression of what the Milky Way might look like seen from above. The colored rings show the rough extent of the fossil galaxy known as Heracles. The yellow dot shows the position of the Sun.(Danny Horta-Darrington

Of the stars examined, a few hundred stars possessed chemical composition and velocities drastically different than the others. The only possible explanation for this peculiarity is that they belonged to a galaxy other than the Milky Way. By a detailed study, the scientists were able to detect the precise location and history of the fossil galaxy.

Though the stars and gas from Heracles constitute a large percentage of the halo, its hidden location inside the Milky Way made its discovery difficult. "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, a member of the research team from Liverpool John Moores University (LJMU).

This experiment examines the centre of the Milky Way much deeper than ever before and provides new insight into the formation of our home galaxy Milky Way. The results serve as testimony to the fact that our galaxy is much more complex than previously thought and could behold numerous such secrets buried within itself.

The study was published in the journal Monthly Notices of the Royal Astronomical Society today and can be accessed here.

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