New Study: Milky Way Swallowed Another Galaxy During Its Early Days

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Scientists studied the early days of our galaxy, the Milky Way, and they say about 13 billion years ago, the universe looked different than it does today. Stars were apparently forming at a rapid rate, which resulted in dwarf galaxies being formed. However, a new study suggests the Milky Way swallowed another galaxy during its formation.

Scientists derived measurements for the positions, brightness and distances of about 1 million stars in the Milky Way within 6,500 light-years of the sun. Scientists gathered this data using the Gaia space telescope, which allowed a team from the Instituto de Astrofísica de Canarias (IAC) to map how the early galaxy looked. The study was published today in the journal Nature Astronomy.

“We have analyzed, and compared with theoretical models, the distribution of colours and magnitudes (brightnesses) of the stars in the Milky Way, splitting them into several components; the so-called stellar halo (a spherical structure which surrounds spiral galaxies) and the thick disc (stars forming the disc of our Galaxy, but occupying a certain height range),” IAC researcher and study author Carme Gallart said in a statement.

Previous research suggested that the galactic halo showed properties of a merger of up to two distinct stellar components. One component seems to have hosted bluer stars than the other. Scientists mapped the movement of the bluer stars and determined that they belonged to a dwarf galaxy known as Gaia Enceladus, which is believed to have been swallowed by the Milky Way in its early days. Nevertheless, scientists don’t know the nature of the red star population and how the merger between Gaia-Enceladus and the Milky Way happened. When the Milky Way swallowed another galaxy during its early stages, the galactic halo which surrounds it was formed.

“Analyzing the data from Gaia has allowed us to obtain the distribution of the ages of the stars in both components and has shown that the two are formed by equally old stars, which are older than those of the thick disc,” IAC researcher and co-author Chris Brook said.

“The final piece of the puzzle was given by the quantity of ‘metals’ (elements which are not hydrogen or helium) in the stars of one component or the other,” IAC researcher and study co-author Tomás Ruiz Lara said.

It seems the Milky Way swallowed another galaxy when stars began forming in the two stellar systems. The galaxy that was swallowed was Gaia-Enceladus, while the other galaxy was the predecessor of Milky Way as we know it. It was four times the mass of Gaia-Enceladus and contained a large proportion of metal. As a result of the collision, the stars from Gaia-Enceladus started moving chaotically, forming the well-known halo around the Milky Way.

“Until now all the cosmological predictions and observations of distant spiral galaxies similar to the Milky Way indicate that this violent phase of merging between smaller structures was very frequent,” IAC researcher and co-author Matteo Monelli said.

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