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Astronomers spot the biggest galaxy ever, and the scale will blow your mind: ScienceAlert

Astronomers spot the biggest galaxy ever, and the scale will blow your mind: ScienceAlert

Earlier this year, astronomers discovered an absolute monster of a galaxy.

Looming about 3 billion light-years away, Alcyoneus is a giant radio galaxy reaching 5 megaparsecs into space. This is 16.3 million light-years long and is the largest known structure of galactic origin.

The discovery highlights our poor understanding of these behemoths and what drives their incredible growth.

But it could provide a path to a better understanding not only of giant radio galaxies, but also of the intergalactic medium that floats in the gaping voids of space.

Giant radio galaxies are another mystery in a universe full of mysteries. They consist of a host galaxy (this is the cluster of stars orbiting a galactic nucleus containing a supermassive Black hole), as well as colossal jets and lobes that erupt from the galactic center.

These jets and lobes interacting with the intergalactic medium act as a synchrotron to accelerate electrons that produce radio emission.

We’re pretty sure we know what’s producing the jets: an active supermassive black hole at the galactic center. We call a black hole “active” when it is absorbing (or “accumulating”) material from a giant disk of material around it.

Not all of the material in the accretion disk that spins into an active black hole inevitably ends up beyond the event horizon. A small fraction of it is somehow channeled from the inner region of the accretion disk to the poles, where it is shot out into space as jets of ionized plasma at velocities a significant percentage of the speed of light.

These jets can travel vast distances before spreading out into giant radio emitting lobes.

The Radiolobes of Alcyoneus. (Oei et al., Astronomy and Astrophysics, 2022)

This process is quite normal. Even the Milky Way has radio links. What we don’t really understand well is why in some galaxies they grow to absolutely massive sizes, on megaparsec scales. These are called giant radio galaxies, and the most extreme examples may be key to understanding what drives their growth.

“If there are host galaxy features that are an important cause of the growth of giant radio galaxies, then the hosts of the largest giant radio galaxies are likely to have them,” the researchers, led by astronomer Martin Oey of the Leiden Observatory in the Netherlands, explained in their paper, which was published in April this year.

“Similarly, if specific large-scale environments exist that are highly favorable for the growth of radio giant galaxies, then the largest radio giant galaxies are likely to reside in them.”

The team began looking for these anomalies in data collected by the LOw Frequency ARray (PROMISES) in Europe, an interferometric network consisting of about 20,000 radio antennas distributed in 52 locations across Europe.

They reprocessed the data through a new pipeline, removing compact radio sources that could interfere with the detection of diffuse radio beams, and correcting for optical distortion.

The resulting images, they say, represent the most sensitive search ever conducted for the lobes of a radio galaxy. Then they used the best pattern recognition tool available to locate their target: their own eyes.

That’s how they found Alcyoneus spewing from a galaxy several billion light years away.

“We have found what is in the projection of the largest known structure made by a single galaxy – a giant radio galaxy with a projected appropriate length [of] 4.99 ± 0.04 megaparsecs. The true correct length is at least … 5.04 ± 0.05 megaparsecs,” they explained.

After measuring the lobes, the researchers used the Sloan Digital Sky Survey to try to understand the host galaxy.

They found it to be a relatively normal elliptical galaxy embedded in a filament of space networkclocked at about 240 billion times the mass of the Sun, with a supermassive black hole at its center with a mass of about 400 million times the mass of the Sun.

Both parameters are actually at the lower end for giant radio galaxies, which may provide some clues as to what drives the growth of the radio lobes.

“Beyond geometry, Alcyoneus and its host are suspiciously ordinary: the total low-frequency luminosity density, stellar mass, and supermassive black hole mass are lower, although similar to those of average giant radio galaxies.” the researchers write.

“Thus very massive galaxies or central black holes are not necessary to grow large giants, and if the observed state is representative of the source over its lifetime, neither is high radio power.”

It is possible that Alcyoneus is located in a region of space that is less dense than average, which could allow it to expand – or that interactions with the cosmic web play a role in the object’s growth.

Whatever is behind it, however, researchers believe Alcyoneus is still growing even bigger, far out in the dark of space.

The study was published in Astronomy and astrophysics.

An earlier version of this article was first published in February 2022.

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