The Absolute Cosmos: Ancient Fossil Galaxy buried deep inside Milky Way found

Astronomers have discovered an ancient fossil galaxy lying in the depths of the Milky Way. The galaxy was found by using the data from Sloan Digital Sky Surveys’ Apache Point Observatory Galactic Evolution Experiment (APOGEE).

Scientists think that it may have collided with our galaxy ten billion years ago when Milky Way was still in its early days. The Milky Way itself is 13.5 billion years old.

The fossil galaxy has been named Heracles by scientists. Heracles was an ancient hero from Greek who in mythology received the gift of immortality when the Milky Way was created.

The remains of Heracles accounts for about one third of our galaxy’s spherical halo.

But if this new fossil galaxy is all around us then why we didn’t see it before? The answer is its location…….deep inside our galaxy.

According to scientists, to discover a fossil galaxy like Heracles, they had to study the chemical makeup and motions of thousands of stars in unprecedented details. This is a hard task to perform for stars in the centre of Milky Way as they are hidden from view by interstellar dust clouds.

APOGEE can penetrate through that dust to see deeper into the heart of our galaxy. It does that by looking at stars in near infrared light which doesn’t get blocked by clouds of dust, rather than visible light which gets obscured by dust clouds.

Over the period of ten years, APOGEE has observed the chemical makeup and velocities of more than half a million stars across the Milky Way, including the stars in the previously dust obscured core.

As scientists explain- in order to find strange/odd stars in heavily populated heart of Milky Way, study of such a large number of stars is required. It is more like finding needles in haystack.

Scientists used APOGEE’s data to separate stars of Heracles from those of the original Milky Way.

Of all the tens of thousands of stars observed, a few of them had different velocities and chemical compositions. By detailed analysis of those stars scientists can identify the precise location and history of Heracles galaxy.

Stars that originally belongs to fossil galaxy roughly account to one third of the mass of our Milky Way galaxy’s halo. This indicates that this ancient collision was a major event in Milky Way’s history, which is quite unusual as most similar massive spiral galaxies had much undisturbed early days.

For more information: Evidence from APOGEE for the presence of a major building block of the halo buried in the inner Galaxy

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