Rewriting history

There is evidence for the GSE merger taking place far back in the Milky Way’s history. However, recent work has questioned whether a massive ancient merger is actually needed to explain the properties of the Milky Way as we see it today, and whether all of the stars originally associated with the GSE are from the same merger event.

In 2020, Thomas led the study that identified wrinkles of stars in the Milky Way, and compared these to simulations of different possible mergers. “We can see how the shapes and number of wrinkles change over time using these simulated mergers. This lets us pinpoint the exact time when the simulation best matches what we see in real Gaia data of the Milky Way today – a method we used in this new study too,” says Thomas. “By doing this, we found that the wrinkles were likely caused by a dwarf galaxy colliding with the Milky Way around 2.7 billion years ago. We named this event the Virgo Radial Merger.”

Since then, Thomas and colleagues have further explored this merger, slowly refining the idea that many of the oddly moving stars and debris in the Milky Way’s inner halo were delivered to our galaxy from a much more recent galaxy collision than the GSE. They have also clarified that the stars originally associated with the GSE may have originated from multiple mergers, some ancient.

“The Milky Way’s history is constantly being rewritten at the moment, in no small part thanks to new data from Gaia,” adds Thomas. “Our picture of the Milky Way's past has changed dramatically from even a decade ago, and I think our understanding of these mergers will continue to change rapidly.

“This result – that a large portion of the Milky Way only joined us within the last few billion years – is a big change from what astronomers thought up until now. Many popular models and ideas about how the Milky Way grows would expect a recent head-on collision with a dwarf galaxy of this mass to be very rare.”

It’s likely that the Virgo Radial Merger brought in a family of other small dwarf galaxies and star clusters with it, which would have all joined the Milky Way at around the same time. Future exploration will reveal which of these smaller objects that were previously thought to be related to an ancient GSE are actually related to a more recent Virgo Radial Merger instead.