A new study of stars in the Milky Way suggests how our Galaxy’s formation and evolution might have been affected by the tidal pull of other passing galaxies. 

Galaxies don’t change much over our lifetime and might seem ordinary, even boring, when compared to the violence of a supernova or the mind-bending strangeness of a black hole. In fact, they’re hotbeds of cosmic chaos, cannibalising each other over hundreds of millions of years.

The Milky Way is one such cannibal. Some stars in the further reaches of our Galaxy move very differently from stars in the main disc, indicating that they were likely stripped off from their host galaxy. However, some stars on the galactic outskirts might originate in the Milky Way itself.

An international team studied fourteen stars from two different overdensities (regions in a galaxy that have a higher concentration of stars), A13 and Triangulum-Andromeda (TriAnd). A13 and TriAnd lie on opposite sides of our Galaxy and are more than thirty thousand light years apart, but their motion may point to them sharing a common origin. The trouble is that nobody can agree on how they formed; scenarios range from their birth within the Milky Way’s disc to disruption of a dwarf galaxy.

To find out more about how A13 and TriAnd might have formed, the researchers obtained spectra for each of the fourteen stars. This allowed them to model properties of the stars in some detail, including the proportions of certain elements such as hydrogen and iron. Just as analysing DNA can allow biologists to identify different individuals and species, analysing spectra and chemical abundances can allow astronomers to determine how a star began its life.

“Different parent populations, such as the Milky Way disc or halo, dwarf satellite galaxies or globular clusters, are known to have radically different chemical compositions,” explains lead author Maria Bergemann, a researcher at the Max Planck Institute for Astronomy in Heidelberg. “So once we know what the stars are made of, we can immediately link them to their parent populations,”

The team then compared the chemical abundances of the stars with those of different stellar populations inside and outside the Milky Way.

Surprisingly, the chemical abundances in the stars from A13 and TriAnd only match abundances from stars within the Galactic disc – in other words, they may have originated in our Galaxy rather than being remnants of another.

This raises the question of how they could have ended up so far away from their birthplace. Bergemann’s team proposes that a dwarf galaxy merging with the Milky Way’s disc could have caused the disc to oscillate up and down, depositing stars far away from the rest of the Galaxy. To test this, they simulated an initially stable disc interacting with a dwarf galaxy. Not only does the disc oscillate vertically, it appears to do so on timescales consistent with the ages of A13 and TriAnd.

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“We showed that it may be fairly common for groups of stars in the disk to be relocated to more distant realms within the Milky Way – having been ‘kicked out’ by an invading satellite galaxy,” says co-author Allyson Sheffield, assistant professor of physics at LaGuardia Community College/CUNY. “Similar chemical patterns may also be found in other galaxies – indicating a potential galactic universality of this dynamic process.”

However, the team only looked at the fourteen brightest stars within the overdensities. Gaining a better understanding of galactic dynamics will require looking at many dimmer stars, as well as looking at stars outside A13 and TriAnd.

“We anticipate that ongoing and future surveys like 4MOST and Gaia will provide unique information about chemical composition and kinematics of stars in these overdensities. The two structures we have analysed already are, in our interpretation, associated with large-scale oscillations of the disc, induced by an interaction of the Milky Way and a dwarf galaxy,” says Bergemann.

The evolution of our Milky Way and other galaxies is complex and not fully understood, but studying regions such as A13 and TriAnd will shed light on an otherwise murky subject. As researchers acquire higher and higher resolution spectra for more and more stars, we will understand more about their origins and about how galaxies evolve.

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