Enormous structure discovered near doomed galaxy group is the longest galactic 'tail' ever seen

A gassy blue ball of a galaxy with a long, cloudy tail trailing behind it to the bottom right — Galaxy group NGC 4839 (white box) slowly merges with its much larger neighbor, leaving a long tail of gas behind it

(Image credit: ESA/XMM-Newton)

Like comets and cats, clusters of galaxies sometimes have long tails. Recently, astronomers have found that those tails can span staggering distances, with one newly discovered tail stretching more than a million light-years long — or longer than 10 Milky Way galaxies lined up side to side.

The discovery began when a team of scientists pointed NASA's Chandra X-ray Observatory at a cluster of galaxies — around 50 galaxies bound together by gravity — called NGC 4839. This group is merging with the much larger Coma Cluster, a dense stellar formation some 340 million light-years from Earth, which contains over 1,000 tightly-bunched galaxies.

Researchers tracked NGC 4839 as it moved through the Coma Cluster, leaving a streak of bright, hot gas in its wake — a "tail". Using X-ray data from Chandra, along with optical data from the Sloan Digital Sky Survey, scientists measured this trail as NGC 4839 moved like a boat slicing through water from the outer reaches of the cluster toward its center. The tail ended was 1.5 million light years long — the longest such structure ever observed, according to NASA.

side by side pictures of the galaxy in blue light and purple light — The image on the left shows an X-ray view of the Coma galaxy cluster taken with ESA's XMM-Newton (blue), along with optical data from the Sloan Digital Sky Survey (yellow). The galaxy group NGC 4839 is located in the lower right of that image. The inset on the right is the Chandra image (purple) of the region outlined by the square. The head of NGC 4839's tail is on the left side of the Chandra image and contains the brightest galaxy in the group and the densest gas. The tail trails to the right.

Currently, the gas in NGC 4839's tail is burning especially bright, giving astronomers a rare opportunity to study it in more detail and learn more about the mechanics of how galaxy clusters merge. As it mixes with cooler gasses in the cluster, the tail will gradually disperse. Eventually the gas will become too faint to see, and NGC 4839 will be completely engulfed in the Coma Cluster. A similar fate awaits our own Milky Way, which is expected to collide with the nearby Andromeda galaxy about 4.5 billion years from now.

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Joanna Thompson is a science journalist and runner based in New York. She holds a B.S. in Zoology and a B.A. in Creative Writing from North Carolina State University, as well as a Master's in Science Journalism from NYU's Science, Health and Environmental Reporting Program. Find more of her work in Scientific American, The Daily Beast, Atlas Obscura or Audubon Magazine.