Two Spiral Galaxies Collided. NASA Captured the Result.

This stunning loop littered with black holes offers a better understanding of what happens when galaxies like ours have accidents.

Published 11 Sept 2018, 11:20 BST
A composite image from the Hubble and Chandra space telescopes shows the ring galaxy known as ...
A composite image from the Hubble and Chandra space telescopes shows the ring galaxy known as AM 0644-741, or the Lindsay-Shapley Ring.
Photograph by NASA

About 300 million light-years from Earth, deep in the constellation Volans, two spiral galaxies spiralled into each other, leaving a brilliant ring of black holes and neutron stars in their wake.

The cosmic smash-up resulted in a what’s known as a ring galaxy, named AM 0644-741, which was captured in a pair of photos and is now featured in a newly published study in the Astrophysical Journal.

When two galaxies collide, there are really almost no collisions at all. Stars and other large objects are so spread out that they rarely smash into each other during such a merger. But the gravitational effects can jostle and rearrange galaxies in very noticeable ways.

“As soon as I looked at optical images of ring galaxies, I was hooked. They are so spectacular that they really make you wonder how nature can produce such amazing shapes,” study coauthor Antonella Fruscione, an astrophysicist at The Chandra X-ray Center, writes in an email.

In this case, a smaller galaxy, possibly the one in the bottom left corner of the image, completely passed through AM 0644-741, creating a gravitational disruption in the form of a density wave that rippled out from the galaxy’s centre, like waves generated from a stone thrown into a pond.

“These waves move some of the orbit of the stars toward the outer regions and, mainly, squeeze the gas in the target galaxy and in the surroundings. The denser gas that results reaches the point in which it makes new stars,” study coauthor Anna Wolter, an astronomer with INAF-Osservatorio Astronomico di Brera in Italy, explains in an email.

The existing stars that were moved, as well as their newborn siblings, spread out and created the circular structure of the newly formed ring galaxy, also named the Lindsay-Shapley Ring after Eric Lindsay and Harlow Shapley, the pair of astronomers that discovered it in 1960.

The animation above shows the ring as seen in x-rays by the Chandra X-ray Observatory and in optical light by Hubble Space Telescope. The bright purple spots that circle the galaxy are ultra-luminous x-ray sources, binary systems which astronomers say are most likely powered by either intermediate-mass black holes or neutron stars. Images like these can help solve that mystery and offer potentially useful information for when the Milky Way galaxy collides with our large spiral neighbour Andromeda in roughly four billion years.

“Collecting a large number of these sources will help us in finding answers to these questions, and in the end to how stars are born, live, and die,” Wolter says.

Beyond the science, the images themselves offer a sense of wonder, even for those who study them for a living: “These kinds of images are what drew me to be an astronomer when I was young,” Fruscione says, “and are still why I am excited by my job.”

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