The Andromeda Galaxy Was the Culprit – The Case of the Missing Galaxy

The Andromeda Galaxy is currently the closest massive galaxy to our own Milky Way. However, astronomers have recently discovered that a third galaxy, now nearly gone, existed in our local group two billion years ago. This family of stars was destroyed eons ago by Andromeda, researchers report.

Evidence for the ancient galactic cataclysm was found in a halo of stellar remains surrounding Andromeda (designated M31), together with a trail of stars leading to a tiny galaxy called M32.

“Astronomers have been studying the Local Group—the Milky Way, Andromeda and their companions—for so long. It was shocking to realize that the Milky Way had a large sibling, and we never knew about it,” said Eric Bell, professor of astronomy at the University of Michigan.

The Andromeda Galaxy and M32
The Andromeda Galaxy and M32, imaged by the Galaxy Evolution Explorer. Andromeda is the closest massive galaxy to the Milky Way, and the two bodies will collide in around four billion years. The small blue fuzzy slot seen below the core of Andromeda is the dwarf galaxy M32. Image by NASA/JPL/California Institute of Technology.

The stars in the halos of galaxies have long been understood to contain remnants of galactic collisions. However, computer simulations were able to show that much of this material surrounding Andromeda was the result of a single collision with a massive body. The ancient family of stars, designated M32p, is thought to have been 20 times more massive than any other body consumed by Andromeda.

Astronomers have long been puzzled by the nature of M32. Unlike any other known galaxy, it is small and compact, but filled with young stars. Normally, the stars in dwarf galaxies like this one all form at around the same time,  so they are typically the same age as each other.

At the time of this ancient galactic collision, the first living cells with nuclei were forming on Earth.

The Milky Way and the Andromeda Galaxy are headed toward each other, and will collide in approximately four billion years.

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