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We know so much about long-extinct creatures like dinosaurs thanks to their fossilized bones that act as time capsules, revealing not only their size and shape, but also clues to their diet, behavior, or even mating patterns. Likewise, astronomers can learn about the Milky Way’s galactic history billions of years in the past by studying stellar relics.
In a new study, researchers describe a newly discovered “fossil galaxy” that has until now been hidden deep within our Milky Way.
Large galaxies like the Milky Way, which contains over 100 billion stars, take time to accumulate matter and grow to their gigantic size. In addition to developing their own stars in galactic nurseries, over time large galaxies also merge with small galaxies, which increases their mass.
Over the course of its 12 billion-year history, the Milky Way has gone through a dozen such mergers, devouring a neighbor’s stars and mixing them into an ever-growing stew of requirements alone. With each galactic merger, the shape, size and motion of our galaxy changed, forming its now iconic spiral.
But it is possible to unwind this spiral and in the process decode the previous mergers of the Milky Way.
Just a week after Dr Diederik Kruijssen of the Center for Astronomy at the University of Heidelberg (ZAH) reported recently identified globular clusters that match the properties of a previously unknown collision with what the team dubbed the ‘Kraken’ galaxy , a new study is now reporting yet another hidden galaxy.
Using data from the Sloan Digital Sky Surveys’ Apache Point Observatory Galactic Evolution Experiment (APOGEE), astronomers found the remains of an ancient collision between the Milky Way and a primordial galaxy. The event, which took place 10 billion years ago, when our galaxy was still in its infancy, is responsible for about a third of the spherical halo of the Milky Way.
“To find a fossil galaxy like this, we had to examine the detailed chemical composition and motions of tens of thousands of stars,” Ricardo Schiavon of Liverpool John Moores University (LJMU) in the UK said in a news release. “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. “
Astronomers nicknamed the galactic fossil Heracles, named after the ancient Greek hero also called Hercules, who was granted the gift of immortality when the Milky Way was created, or so the myth goes.
To distinguish the stars of Heracles from all the billions of objects in the Milky Way, the astronomers had to measure the chemical composition and speeds of the stars using APOGEE. A few hundred of these stars had radically different chemical compositions and speeds from all others. The only reasonable explanation is that these objects belonged to a galaxy other than the Milky Way.
“These stars are so different that they could only come from another galaxy. By studying them in detail, we could trace the precise location and history of this fossil galaxy “, Danny Horta of LJMU, lead author of the new study published in Royal Astronomical Society Monthly Noticeshe said in a statement.
This is just the tip of the iceberg. In the future, the fifth phase of the Sloan Digital Sky Survey and his “Milky Way Mapper” will measure the spectra for a number ten times more stars in the entire galaxy. Who knows what cosmic fossils astronomers will then discover.
“As our cosmic home, the Milky Way is already special to us, but this ancient galaxy buried within it makes it even more special,” says Schiavon.
And as you may be wondering, the next merger is planned for 2.5 billion years from now with the neighboring Andromeda Galaxy.
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