Author: Lisa Grossman / Source: Science News
In its younger days, the Milky Way devoured a smaller galaxy, and stars from the hapless victim still roam the skies today to tell the tale, a new study finds.
“This is a major event in the history of the galaxy,” says astronomer Amina Helmi of the University of Groningen in the Netherlands. “We’re really starting to probe the ancestors of the Milky Way.”
Helmi and her colleagues analyzed the speeds and positions of tens of thousands of stars in the Milky Way within about 33,000 light-years of the sun, using data from the European Space Agency’s Gaia space telescope (SN Online: 5/9/18). A group of about 30,000 stars seem to be moving backward, the team reports October 31 in Nature. Instead of rotating around the galactic center with the sun and the rest of the stars in the Milky Way’s bright disk, these stars travel in the opposite direction.
“That was the first hint,” Helmi says. “When stars move the opposite way, that already tells you that they basically didn’t form in the same place as the majority of the stars in our galaxy.”
The second hint came from looking those stars up in a catalog from the Apache Point Observatory Galactic Evolution Experiment, or APOGEE, which uses the spectrum of light that stars emit to infer their chemistry and ages. The backward-moving stars had fewer heavy elements than stars like the sun. That suggests that they formed earlier in the universe’s history, before there was time for massive stars and supernovas to spread heavy elements around the galaxy (SN: 10/1/16, p. 25).
That chemistry clinched it, Helmi says. “It was just so clean. You immediately knew these stars really formed elsewhere.”
…
The post The Milky Way feasted on a smaller galaxy 10 billion years ago appeared first on FeedBox.