Astronomers spy "Bullet Galaxy" piercing another galaxy

A distant galaxy penetrated another group of galaxies like a bullet, astrophysicists report, in a cosmic crash some 1.4 billion light-years away.

Observations from Europe's XMM-Newton X-ray space telescope disclose that the bullet galaxy pierced through a galaxy cluster called Abell 4067 at 814 miles (1,310 kilometers) per second.

That bullet weigh up about 200 trillion times as much as Earth, report starwatchers Gayoung Chon and Hans Böhringer of the Max Planck Institute for Extraterrestrial Physics in Garching, Germany. Gassy debris tracks from the impact, they write in an upcoming Astronomy & Astrophysics journal study.

The cosmic impact turned up in a study of 900 distant galaxy clusters, so discovering the merger "is a nice surprise," Chon says by email

A first "bullet cluster" exposed how stars and gas (red) divide from dark matter (blue) in galaxy collisions
When clusters of galaxies collide, researchers get a rare glimpse into how these cosmic smash-ups unfold, including how much these distant galaxies in fact weigh.

In 2008, study of an even faster moving "bullet cluster" presented cosmologists evidence that dark matter, a secretive and unseen substance that makes up most of the total matter in the universe, rings most galaxies. While the bullet galaxy left a track of hot gas, the hidden bulk of its mass—dark matter—continued on a distinct trajectory.

As the new bullet cluster is less gigantic and the fusion slower, weighing its dark matter could be harder, Chon says.

The newly found bullet cluster should help respond to questions about the mass of distant galaxies and how they act when they crash together. (Of course, since light takes time to travel through galaxies, the collision at Abell 4067 really occurred 1.4 billion years ago, and we're just seeing it now.)

Our own Milky Way galaxy possibly faces a galactic smash with the neighbouring Andromeda galaxy within four billion years, meaning that someday, these mergers will be more than a matter of educational curiosity. 

A cool, dense galaxy (white, center) penetrates a cluster of galaxies called Abell 4067.

Closer study of the "bullet" should help disclose how much gas surrounds the ancient, compact galaxy, the researchers suggest. And they hope to exactly image the shock wave spawned by the smash.

What's more, Chon says, "we have just been presented a seven-times-deeper XMM[-Newton] study of this object, so we will have even more facts on the merger behaviour."


Galactic collision

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