The Milky Way feasted on a smaller galaxy 10 billion years ago

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.”

Comparing the observations with computer simulations, Helmi’s team concluded that 10 billion years ago, the Milky Way collided with a smaller galaxy — perhaps about 20 to 25 percent the size of the Milky Way at the time — containing the equivalent of 600 million stars the mass of the sun. 

That collision may explain why the Milky Way is split into different components: a thin disk of young stars organized into spiral arms, a thick disk of much older stars surrounding the thin disk, and a spherical halo of stars whose trajectories take them far from the disks. Many of the present-day halo stars came from the gobbled-up galaxy.

The team named the doomed galaxy Gaia-Enceladus. In Greek mythology, the giant Enceladus was the son of Gaia (the Earth) and Uranus (the sky). Buried under Mount Etna in Italy, the legend goes, Enceladus is responsible for the region’s earthquakes. “The analogy is that Enceladus the galaxy is buried in the Milky Way, it was buried in the Gaia data, and it was the cause of a lot of shaking and puffing up of the disk of the Milky Way at that time,” Helmi says.

Signs of reverse-moving and chemically distinct stars had been seen in earlier galactic surveys, says astronomer Julio Navarro of the University of Victoria in Canada, who was not involved in the new work. He identified about 120 of the foreign stars in a paper in 2011. And other groups have used Gaia data and APOGEE data separately to identify the same group of odd stars, which they dubbed the “Gaia sausage.”

Seeing both sets of data come together with thousands of stars “is quite comforting,” Navarro says. “The details still need to be ironed out,” he says. “The main idea that the galaxy swallowed a pretty heavy satellite quite a while ago, I think that’s probably beyond doubt.”

The stars that once belonged to Gaia-Enceladus are like a fossilized record of what galaxies were like 10 billion years ago. Studying them can help reveal details of ancient galaxies, and help understand how galaxies form in general, says astrophysicist Paula Jofré of Universidad Diego Portales in Santiago, Chile. Jofré has previously used stars’ chemical contents to identify family trees among stars near the sun (SN: 10/13/18, p. 24).

“Now that this galaxy has been identified, I think it is interesting to study as many chemical elements as possible and see if we can reconstruct its history using evolutionary trees as we did for the solar neighborhood,” she says.