2 Billion Years Ago Andromeda Swallowed Another Galaxy – These Stars Are All That’s Left

Someday, the Andromeda Galaxy will merge with our Milky Way – and it won’t be the first time Andromeda has swallowed another galaxy and kept the stars for itself.

The nearby Andromeda Galaxy, like most galaxies, is made up of more than just the familiar disk with its graceful arms. A spherical halo of stars and gas surrounds the disk – and many of the stars in that halo are remnants of other galaxy, engulfed in a merger two billion years ago.

A team of astronomers led by NOIRLab astronomer Arjun Dey published their findings in a recent paper in The Astrophysical Journal.

What’s new

Dey and his colleagues measured the motion of about 7,500 stars in the halo of matter around the nearby Andromeda Galaxy. When astronomers plotted how large groups of stars moved in different areas of the galaxy, astronomers noticed patterns and structures in the movement. And those patterns suggested that many of Andromeda’s stars were born in an entirely different galaxy.

Here in our Solar System, we can see how a moon orbits a planet and get some clues as to whether the moon formed from debris that already orbited the planet or was a stray object captured by the planet’s gravity. Likewise, the way many of Andromeda’s stars move around the galaxy’s center appears to match what computer simulations predict after an ancient cosmic merger, in which Andromeda absorbed a smaller galaxy.

Today, all that remains of this smaller galaxy are thousands of stars in Andromeda whose orbits indicate their origins.

Each point in this image represents a single star in the Andromeda Galaxy. Red dots are stars that are moving away from us, blue dots are stars that are moving towards us, and green and yellow dots are more or less keeping the same distance.

KPNO/NOIRLab/AURA/NSF/E. Slawik/D. by Martin/M. Zamani

Here is the background

For years astronomical theories have predicted that the motions of stars should reveal ancient collisions between galaxies, but this recent study is the first time astronomers have managed to do this kind of “cosmic paleontology” on a galaxy outside our own Milky Way. Rather than looking to fossils and rock layers to understand a planet’s history, astronomers study the motion and chemical composition of stars to understand the history of an entire galaxy.

“It’s amazing that we can look up at the sky and read billions of years of another galaxy’s history as written in the motions of its stars,” said NOIRLab astronomer Joan Najita, co-author of the recent study, in a press release. “Each star tells part of the story.”

While recent observations of the Andromeda Galaxy are the first time that the motion of stars has helped to reconstruct a galactic merger, there are other ways to unravel a galaxy’s complicated history. In a 2021 study, a different team of astronomers studied the chemical composition of red giant stars in the Large Magellanic Cloud (a dwarf galaxy that orbits the Milky Way) and found that at least one densely packed star cluster in the Large Magellanic Cloud it’s basically a stellar fossil left over from an even smaller galaxy that the Large Magellanic Cloud once swallowed. It’s a common event in the life of a galaxy.

why does it matter

Studying how stars migrated to Andromeda during its relatively recent merger could shed some light on our own galaxy’s backstory — and its ultimate fate.

“Galaxies like M31 (Andromeda) and our Milky Way are built from the building blocks of many smaller galaxies throughout cosmic history,” says Dey in a recent statement. Most of the stars in our galactic halo also formed in another galaxy, which merged with the Milky Way between eight and ten billion years ago.

“Our emerging picture is that the history of the Andromeda Galaxy is similar to that of our own galaxy, the Milky Way,” said Sergey Koposov, an astrophysicist at the University of Edinburg, in a recent statement. “The inner halos of both galaxies are dominated by a single immigration event.” In other words, most of the imported stars that Dey and his colleagues found in Andromeda came from just one other galaxy that merged with Andromeda — not a series of small mergers. And the history of our Milky Way appears to be quite similar in that regard.

And in about five billion years, we’re going to collide and merge with Andromeda.

What is the next

Dey and his colleagues say the 7,500 stars in their recent study were just a sample; now they plan to survey all visible stars in Andromeda’s galactic halo.

They will use the same instrument they used for the recent study: the Dark Energy Spectroscopic Instrument mounted on the fifty-year-old Mayall Telescope at Kitt Peak National Observatory. The instrument was originally designed to map galaxies and quasars to help astronomers measure how fast the universe is expanding.

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