Collision helped make the Milky Way -- and now we know when

January 13, 2020

Thanks to some astrophysical sleuthing, researchers have pinpointed an early galactic merger that helped shape the Milky Way.

The merger -- a collision, actually -- happened 11.5 billion years ago. That's when a small galaxy called Gaia-Enceladus slammed into what then existed of the Milky Way, Earth's home galaxy, which is about 13.5 billion years old.

"We know today that the Milky Way was formed by the merger of many small galaxies. This is the first time we have been able to determine when such a merger happened," said Sarbani Basu, professor and chair of astronomy at Yale and co-author of a new study reporting the discovery. "This is an important step in understanding when the Milky Way accreted, or collected, its mass."

The study appears Jan. 13 in the journal Nature Astronomy. Dozens of astronomers from around the world, led by the University of Birmingham in the U.K., conducted the work. Yale graduate student Joel Ong is also a co-author.

The research team followed the life story of a single, bright star in the Indus constellation, visible today from Earth's southern hemisphere. The scientists said this star, nu Indi, was already orbiting inside the Milky Way prior to the Gaia-Enceladus collision, which unfolded over millions of years. As the merger progressed, it altered nu Indi's orbit around the center of the Milky Way, providing a marker for when the merger happened. (Stars have orbits, just as planets do.)

"My role was to determine the age of the star (nu Indi) using seismic data," Basu said. "Like many low-mass stars, this star pulsates, or quakes, continuously. The quakes can be described as a series of tones and overtones."

Basu and her colleagues calculated "frequencies" from nu Indi's tones and overtones. Those frequencies, in turn, indicated the star's physical structure and properties. From there, the researchers were able to gauge nu Indi's stage of development, factor in its brightness, and estimate its age.

Knowing nu Indi's age provided a limit for when the merger could have taken place, the researchers said.

Some of the world's latest technology aided researchers in their detective work. They got data on nu Indi's quakes from NASA's Transiting Exoplanet Survey Satellite (TESS). Launched in 2018, TESS is surveying stars across most of the sky to search for planets orbiting those stars and to study the stars themselves. The researchers also used information collected from the European Space Agency (ESA) Gaia Mission.

University of Birmingham astrophysicist Bill Chaplin, lead author of the study, said determining the natural oscillations of stars -- called asteroseismology -- is a way to better understand the history of stars and the environments in which they formed.

"This study demonstrates the potential of asteroseismology with TESS, and what is possible when one has a variety of cutting-edge data available on a single, bright star," Chaplin said.

Yale University

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