Galactic Archaeologists Find Stars in our Milky Way That Survived an Ancient Galaxy Merger

Thursday, 01 November 2018 - 1:12PM
Astronomy
Space
Thursday, 01 November 2018 - 1:12PM
Galactic Archaeologists Find Stars in our Milky Way That Survived an Ancient Galaxy Merger
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Credit: NASA, ESA, STScI, R. Gendler, and the Subaru Telescope (NAOJ)
There are some awesome job titles out there, like "Digital Prophet" and "Dean of Pizza, but "galactic archaeologist" probably takes the cake. It brings to mind Indiana Jones in a space suit, or at least ancient alien civilizations. But even a Prometheus-style planet full of alien artifacts is too small-scale for a real galaxy-level archaeology—instead, the real focus is treating the Milky Way (and beyond) as a giant fossil record. Most recently, an international team of scientists led by Amina Helmi of the University of Groningen identified one of the major galactic collisions (or "mergers") that shaped the Milky Way as we know it.

The merger occurred roughly 10 billion years ago and involved the Gaia-Enceladus galaxy, which slammed into the Milky Way and brought along a whole slew of young stars. To really uncover the truth about the merger, however, Helmi's team had to find 'fossils' from it. They knew that there was a collection of stars that seemed to have a different composition from the others in the Milky Way, but in the wake of the Gaia satellite's giant dump of data on over 1.7 billion stars, they were finally able to piece together the full picture: most of the outer 'halo' of the Milky Way comes from Gaia-Enceladus. According to Helmi: "We expected stars from fused satellites in the halo. What we didn't expect to find was that most halo stars actually have a shared origin in one very large merger."

By looking at the chemical signatures and trajectories of the stars, Helmi's team was able to easily tell the Gaia-Enceladus stars from the 'native' ones. "The youngest stars from Gaia-Enceladus are actually younger than the native Milky Way stars in what is now the thick disk region," said Helmi. "This means that the progenitor of this thick disk was already present when the fusion happened, and Gaia-Enceladus, because of its large size, shook it and puffed it up."

If you want to see what a galaxy merger looks like from a God's-eye perspective, check out this simulation created by the research team (below). It models the Milky Way's merger with the Small Magellanic Cloud, a dwarf galaxy.

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