Astronomers discover the largest-ever galaxy thanks to a “stroke of luck”

It is larger than 100 Milky Ways.
Chris Young
A combined radio-infrared image of Alcyoneus.Universiteit Leiden

Astronomers discovered a gargantuan radio galaxy that stretches a span of 16 million light-years, making it the largest ever observed, a press statement reveals.

The galaxy, called Alcyoneus after a Gigante (giant) of Greek mythology, is located roughly 3 billion light-years away from Earth. And it was discovered thanks to a "stroke of luck" by an international team of researchers from the U.K, France, and the Netherlands. 

In a study published in the journal Astronomy & Astrophysics, the researchers outlined how, using data from the Low-Frequency Array (LOFAR) network of radio telescopes, they detected two enormous plumes of plasma coming from a supermassive black hole. These two plumes were 16 million light-years long (more than one hundred Milky Ways in length), making them the largest-ever observed structure of galactic origin.

Last year, LOFAR was responsible for two world firsts when it detected warm matter emanating from a black hole in a galactic system called Nest200047, and when data from the telescope network were used to create the most detailed images of deep space yet using radio waves.

A "giant" galaxy hiding in the sky

In the case of the new discovery, LOFAR detected plasma that was blasted into space by the Alcyoneus galaxy's black hole in the form of two plumes. When matter is sucked into a black hole in a radio galaxy, it violently releases plasma, which includes the materials required for new stars to form. This plasma emits radio light, which allowed for the galaxy to be detected by LOFAR.

"Despite [Alcyoneus'] mind-boggling distance [from Earth], the giant looms as large in the sky as the Moon — an indication that the structure had to have a record length," the researchers said in their statement. "The fact that the radio eyes of the LOFAR telescope only saw the giant just now, is because the plumes are relatively faint. By reprocessing a set of existing images in such a way that subtle patterns stood out, the scientists were suddenly able to spot the giant."

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Alcyoneus might provide new insight into the formation and structure of our universe, the astronomers explained in their statement. Specifically, they believe that the discovery of such a massive galaxy might reveal clues about the mysterious nature of the cosmic web, which is believed to connect all galaxies via a network of threads and nodes. The researchers also say they are not certain what caused Alcyoneus to grow into such an enormous galaxy, and they will continue to carry out observations in order to gain a better understanding of the galactic giant.

Study Abstract:
We discover what is in projection the largest known structure of galactic origin: a giant radio galaxy with a projected proper length of 4.99±0.04 Mpc. The source, named Alcyoneus, was first identified in low-resolution LOFAR Two-metre Sky Survey images from which angularly compact sources had been removed. Being an extreme example in its class, Alcyoneus could shed light on the main mechanisms that drive radio galaxy growth. We find that - beyond geometry - Alcyoneus and its host galaxy appear suspiciously ordinary: the total low-frequency luminosity density, stellar mass and supermassive black hole mass are all lower than, though similar to, those of the medial giant radio galaxy (percentiles 45±3%, 25±9% and 23±11%, respectively). The source resides in a filament of the Cosmic Web, with which it might have significant thermodynamic interaction. At 5⋅10−16 Pa, the pressures in the lobes are the lowest hitherto found, and Alcyoneus therefore represents one of the most promising radio galaxies yet to probe the warm-hot intergalactic medium.

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