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

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

About 3 billion light-years away, Alcyoneus is a giant radio galaxy that reaches 5 megaparsecs into space. That’s 16.3 millions light-years long, and constitutes 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 just of giant radio galaxies, but of the intergalactic medium that drifts in the yawning voids of space.

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

These jets and lobes, which interact with the intergalactic medium, act as a synchrotron to accelerate electrons that produce radio emissions.

We’re pretty sure we know what produces the jets: an active supermassive black hole at the galactic center. We refer to a black hole as “active” when it is consuming (or “accreting”) material from a giant disk of material around it.

Not all material in the accretion disk spinning into an active black hole inevitably ends up beyond the event horizon. Somehow, a small fraction is sent from the inner region of the accretion disk to the poles, where it is ejected into space as jets of ionized plasma, at speeds of a significant percentage of the of the light

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

This process is quite normal. Even the Milky Way has radio lobes. What we don’t really have a good handle on is why, in some galaxies, they grow to absolutely gargantuan sizes, on megaparsec scales. These are called giant radio galaxies, and the most extreme examples could be key to understanding what drives their growth.

“If features of the host galaxy exist that are an important cause for the growth of giant radio galaxies, the hosts of the largest radio giant galaxies are likely to possess them,” the researchers, led by astronomer Martijn Oei from the Leiden Observatory, in the Netherlands. explained in his document, which was published in April this year.

“Similarly, if particular large-scale environments exist that are highly conducive to the growth of radio giant galaxies, then the largest radio giant galaxies are likely to reside there.”

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

They reprocessed the data using a new pipeline, removing compact radio sources that could interfere with diffuse radio lobe detections and correcting for optical distortion.

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

That’s how they found Alcyoneus, which would be emitting from a galaxy several billion light years away.

“We discovered what is projected to be the largest known structure made by a single galaxy: a giant radio galaxy with a projected proper length. [of] 4.99 ± 0.04 megaparsecs. The true proper length is at least … 5.04 ± 0.05 megaparsecs.” they explained.

Once they measured the lobes, the researchers used the Sloan Digital Sky Survey to try to understand the host galaxy.

They discovered that it is a fairly normal elliptical galaxy, embedded in a filament of the cosmic webclocking in around 240 billion times the mass of the Sun, with a supermassive black hole at its center around 400 million times the mass of the Sun.

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

“Beyond the geometry, Alcyoneus and its host are suspiciously common: the total low-frequency luminosity density, stellar mass, and supermassive black hole mass are lower than, though similar to, those of galaxies in radio media giants”. the researchers wrote.

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

It could be that Alcyoneus is sitting in a region of space with a lower-than-average density, which could allow it to expand, or that interaction with the cosmic web plays a role in the object’s growth.

Whatever is behind it, however, researchers believe that Alcyoneus is still getting bigger, far out in the cosmic darkness.

The research was published in Astronomy and Astrophysics.

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