A glimpse into the youth of our galaxy

How do galaxies like our Milky Way system form? They grow by merging with other islands of worlds in their neighbourhood, thereby increasing their mass and thus the number of their stars. Our Milky Way system, scientifically known as the Galaxy, was no exception when it formed around 12 to 13 billion years ago - a few hundred million years after the Big Bang. Now Khyati Malhan and Hans-Walter Rix, two researchers from the Max Planck Institute for Astronomy in Heidelberg, have succeeded in tracking down two of the earliest of these mergers. They named the stellar populations, totalling around six million stars, Shiva and Shakti.

The two scientists discovered Shiva and Shakti by using data from the European astronomical satellite Gaia and the Sloan Digital Sky Survey (SDss). At first glance, it is very surprising that traces of galaxy mergers can still be found shortly after the Big Bang, because the stellar populations and the collections of gas and dust carried in the world islands also mixed in the growing galaxy. However, the stars involved retain some basic properties that enable them to be recognised. They can be traced back to the movements and directions of the world islands merging with the Milky Way system.

The stars from the two early galaxies each have characteristic values for their angular momentum and kinetic energy. These remain even when they move in the gravitational field of our galaxy. With the help of the huge data set of the Gaia space telescope, larger groups of stars can be identified from its star catalogue with around one and a half billion entries through complex data filtering. They then each have the same unusual values for kinetic energy and angular momentum that distinguish them from the majority of stars in our Milky Way system.

Another clue is provided by the chemical fingerprint, especially the content of elements that are heavier than hydrogen and helium and are simply referred to as metals in astrophysics. This content is called "metallicity". If you are looking for processes that took place in the youth of our universe and thus also of the Milky Way system, you have to look for very old stars. They are characterised by a low metallicity, i.e. a low proportion of heavy elements, and are referred to as metal-poor. Our sun, on the other hand, which "only" formed around 4.5 billion years ago, is very metal-rich.

To find this out, Malhan and Rix compared the stars selected from the Gaia data with the detailed spectroscopic data from the SDss sky survey to determine their chemical composition. When analysing the data, two larger groups with a total of six million members stood out, each with similar values for composition, kinetic energy and angular momentum. First author Malhan named them Shakti and Shiva. Shiva is one of the main deities of Hinduism and Shakti is a female cosmic force that is often depicted as Shiva's consort.

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