Milky Way neighbours “ripped out” by colliding galaxy

Analysis finds stars in satellite galaxies were once part of the our stellar home. Lauren Fuge reports.

The Keck telescope, helping to resolve the origin of the Milky Way's neighbours.
The Keck telescope, helping to resolve the origin of the Milky Way's neighbours.

Stars currently orbiting the Milky Way were violently ripped from our own galaxy by an invading satellite galaxy, astronomers have discovered.

When galaxies pass close by to each other, massive gravitational forces fling stars, dust and gas around like a giant cosmic blender. These interactions can dramatically distort a galaxy’s structure and shape, and even influence its future evolution. The Milky Way has led an active and often violent life with many close gravitational shaves; its iconic spiral structure may even be the result of one such tussle.

Now an international team of astronomers, led by Maria Bergemann from the Max Planck Institute for Astronomy in Germany, has studied two groups of stars in the stellar halo that encircles the Milky Way’s star-studded spiral disc. The chemical composition of these stars was found to closely match those in the galactic disc, providing compelling evidence that they have been evicted from their original birthplace in the Milky Way.

The paper is published in the journal Nature.

These stars reside within two regions in the halo, called A13 and Triangulum-Andromeda. One is located about 14,000 light-years above the plane of the Milky Way’s disc, and the other 14,000 light-years below.

Using two of the largest telescopes in the world — the 10-metre Keck telescope in Hawaii and the 8-metre Very Large Telescope in Chile — the team analysed the spectrum of light emitted by the stars and thus determined the abundances of chemical elements produced in each.

Co-author Luca Casagrande of the Australian National University explains: “We would expect to see certain chemical signatures if these stars were to originate from a disrupted globular cluster, or a satellite galaxy cannibalised by the Milky Way.”

But the stars have the same chemical fingerprints as ones in the Milky Way’s disc, leading the researchers to believe that they were torn from it by tidal interactions caused by a dwarf galaxy that came too close for comfort.

Such interactions are common. The Milky Way is surrounded by a number of smaller dwarf galaxies and has been influenced by their gravitational pull throughout its life.

According to Casagrande, knowing that these stars originate from the disc “is a game changer in understanding the evolution of the Milky Way. It means that tidal interactions of satellite galaxies with the Milky Way are capable of perturbing the disc as a whole, sending waves across it, and evicting stars in regions we didn't think it was possible.”

Computer simulations modelling this interaction showed that the stars were ripped from our galaxy around six billion years ago, give or take two billion years.

With a range of revolutionary stellar surveys currently underway — such as the Australian GALAH survey, and the Gaia, K2 and the Transiting Exoplanet Survey (TESS) satellites — results like these are sure to soon be dwarfed by new understanding.

Lauren Fuge is an Adelaide-based author and science communicator.
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