Scientists Stumble Upon Largest Galaxy Ever in 'Stroke of Luck'

Astronomers have discovered the largest giant radio galaxy ever stretching out 5 megaparsecs—or 16.3 million light-years—into space.

Located around 3 billion light-years from Earth, the galactic structure, which has been named Alcyoneus, was discovered in what the lead researcher described as a "stroke of luck."

The name Alcyoneus derives from Greek mythology. Alcyoneus is one of the Giants that battle the Olympian Gods. He is often considered the opponent of Hercules.

The giant radio galaxy measures over 160 times the diameter of our galaxy, the Milky Way, and is composed of two massive plasma plumes emitted from an ordinary-seeming galaxy. It is not yet known how a single normal-looking galaxy could produce such massive outflows.

Leiden University PhD student Martijn Oei is the lead author of a paper accepted for publication in the journal Astronomy & Astrophysics. He told Newsweek: "As far as we know at this moment, the remarkable thing about this galaxy is that it is actually so unremarkable in all aspects we've checked until now.

"Its outflows are larger than any we've seen before but we don't know yet why!"

In addition to this mystery, Oei said that the discovery came as a happy accident for the team, which was examining data collected by the LOw Frequency ARray (LOFAR) network of around 20,000 radio antennas located across Europe, comprising the world's largest radio telescope.

Oei said that he and his Ph.D. supervisors were investigating the distribution of the "cosmic web" material filaments that stretch between galaxies in radio waves when he stumbled upon this massive galactic structure.

He said: "The discovery of Alcyoneus was serendipitous! At the time, I had only just learned about how large the largest radio galaxy was, so I wasn't at all an expert in this.

"Still I had a feeling that this was potentially special. I ran the numbers on an online calculator to get a first approximate size out—and it was 5 megaparsecs."

It was paper co-author Martin Hardcastle, one of the world's leading experts in radio galaxies, that told Oei the significance of what he had found. "He replied to my e-mail 'congratulations, this is the largest radio galaxy ever found!'"

What Is A Giant Radio Galaxy?

A radio galaxy is a galactic structure comprised of a host galaxy with a supermassive black hole at its heart, and astrophysical jets of ionized gas or plasma that are blown out of the central region—or active galactic nuclei (AGN)—as material falls into the black hole.

As material is fed, or accreted, to the central supermassive black holes from a surrounding accretion disc, some of it escapes consumption and slips from the inner region to the poles from where it is blasted into space as a jet of plasma, near-light speeds. As the jets travel they spread out taking a lobe like an appearance.

This is fairly common and happens in many galaxies, including the Milky Way. What is less clear is how these jets grow to such tremendous sizes.

When these jets reach a scale of megaparsecs (Mpc), they are called giant radio galaxies because they interact with the intergalactic medium that surrounds galaxies accelerating electrons creating radio emissions.

Oei told Newsweek that the answer to this mystery may lie in the environments of such galaxies: "What environment the galaxy lives in could maybe explain its giant growth! There is an idea that in low-density environments, the plumes can more easily grow as they feel less resistance from the surrounding medium."

The Leiden University Ph.D. student and his colleagues will now investigate if this is the case, but doing so will involve the study of galaxies closer to home than Alcyoneus. He said: "We are now investigating this possibility, however with radio giants closer than Alcyoneus, where reliable density analyses are already possible.

"For this kind of investigation on Alcyoneus, we'll have to wait a few years until new facilities become operational!" he said.

"Galaxies are not just peaceful collections of stars and dust, but that many also feature violent, spectacular processes—like glowing jet streams and plumes that reach far out into the depths of space, many millions of lightyears, or 100 times their own size!"

Largest Galaxy
Three images of the largest galaxy ever discovered by astronomers in infrared, radio, and optical wavelengths. The galaxy in question Alcyoneus, spans a whopping 16.3 million light years. Martijn Simon Soen Liong Oei/LOFAR