“Cosmic ring of fire” tells tale of ancient intergalactic collision

Astronomers have discovered a “cosmic ring of fire” – a galaxy shaped like a gigantic donut. This type of galaxy is extremely rare, especially for its age, and it was likely caused by a collision with a smaller galaxy that can still be seen lingering near the scene of the accident.

The ring-shaped galaxy is named R5519, and it’s a long way from Earth, in both space and time – 10.8 billion light-years away. It’s roughly the same size as the Milky Way, but the hole in the middle spans more than 30,000 light-years. For comparison’s sake, if our home galaxy had a hole that big at its center, it would extend past the solar system.

The ring itself is alight with a great density of stars, after the collision triggered a wave of new star formation. In fact, R5519 is producing new stars at a rate 50 times greater than the Milky Way.

The team says that this unusual shape was probably caused by a violent encounter with another galaxy. Once upon a time, R5519 would have had a thin disk shape, until a smaller galaxy plowed right through the center of it. The gravitational effects of the collision would have sent the stars rippling outwards, leaving the center mostly empty.

And sure enough, in Hubble images a small galaxy can be seen nearby, providing the smoking gun needed for this hypothesis.

That would make R5519 an extreme rarity. Most ring galaxies get their structure from internal processes, when the huge inner “bar” of stars and material becomes unstable and effectively sweeps the inner region clean. But rings caused by collisions are around 1,000 times rarer.

And this one is in a class of its own. It’s the first collisional ring galaxy to be found at such a long distance, which means it formed that structure almost 11 billion years ago. The problem is, it’s thought that galaxies wouldn’t have been advanced enough to have thin disks that early in the evolution of the universe.

“In the case of this ring galaxy, we are looking back into the early universe by 11 billion years, into a time when thin disks were only just assembling,” says Kenneth Freeman, co-author of the study. “For comparison, the thin disk of our Milky Way began to come together only about nine billion years ago. This discovery is an indication that disk assembly in spiral galaxies occurred over a more extended period than previously thought.”

This isn’t the only recent study to raise questions about how early galactic structures arose. Earlier this week astronomers discovered a 12.3 billion-year-old galaxy with a disk shape – which is 4.5 billion years earlier than they were thought to have formed.

It seems we still have a lot to discover about the early years of the universe.

The research was published in the journal Nature Astronomy. An animation of the collision can be seen in the video below.

“Cosmic ring of fire” animation

Source: ASTRO 3D

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