A group of five galaxies retained a huge scar from a collision with a sixth

(ORDO NEWS) — Astronomers have detected an exceptionally long stream of hydrogen emanating from Stephan’s Quintet, the famous compact group of galaxies.

It looks like it was formed due to the passage of another galaxy or collision with it more than a billion years ago.

Stephen’s Quintet lies in the constellation Pegasus, about 300 million light-years away. This group includes five galaxies, four of which form a compact group and continuously interact with each other.

Thanks to its unusual structure and spectacular appearance, Stephen’s Quintet has been the subject of active observation since its discovery at the end of the 19th century. Almost all famous telescopes, including Hubble and James Webb, have explored it.

Regular observations of galaxies were carried out using the new Chinese FAST radio telescope , which was launched just a couple of years ago and boasts the largest “dish” in the world: its diameter is 500 meters.

According to Cong Xu and his co-authors, FAST was able to look at the details of the Stephen Quintet that were still unknown. And the main find was a huge stream of neutral atomic hydrogen.

This component of galaxies can be called the most mobile: it easily breaks away under the influence of gravitational forces.

Thanks to this, atomic hydrogen makes it possible to trace the interactions experienced by celestial objects in the past.

The structure seen in Stefan’s Quintet turned out to be one of the largest known, stretching for two million light-years, and possibly more – according to scientists, the edge of the stream went beyond FAST’s field of view.

Cong Xu and his colleagues believe that the hydrogen trail was created by the interaction of Stephan’s Quintet with the nearby and fast galaxy NGC 7320a, which is moving at an impressive speed of more than 6,700 kilometers per second.

She could pass close to the group, pulling a “tail” of hydrogen out of it. According to another scenario, the trail arose due to a head-on collision of a yet unknown galaxy with one of the members of the Quintet.

This could lead to the expulsion of large amounts of atomic hydrogen, and the long stream is only the densest part of the resulting cloud.

In any case, judging by the speed of hydrogen movement, it happened about one and a half billion years ago, which came as a surprise to scientists.

Existing models show that such structures of light and mobile gas should dissipate on scales of hundreds of millions of years. How and why this stream managed to survive for more than a billion years is not yet clear.

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