This year, astronomers discovered an absolute monster of a galaxy.
Alkyoneus is a huge ray of the hidden galaxy reaching a distance of 5 megaparsecs. That is 16.3 million light years long, and constitutes the best-known structure of galactic origin.
Our discovery highlights the poor understanding of these giants, and what drives their incredible growth.
But it could provide a path to a better understanding, not only of radio galaxies, but of the intergalactic medium that flows in the yawning space.
Giant radio galaxies are yet another mystery in a universe full of mysteries. The host galaxy (this is the cluster of stars orbiting the galactic core containing the supermassive black hole), as well as the colossal bursts and lobes that erupt from the galactic center.
These bursts and lobes, entangled with the intergalactic medium, act as synchrotrons to enhance the radio emission of electrons.
We’re pretty sure we know what’s going on: the active supermassive black hole at the center of the galaxy. We say that a black hole is ‘active’ when it spews (or ‘accretes’) matter from the massive material around it.
Not all material in the accretion disk that flows into an active black hole necessarily ends up outside the event horizon. Some of its fraction, in a funnel-like manner, from the inner region of the accretion disk to the poles, where it bursts into space in the form of an ionized plasma, runs at a significant speed at the speed of light.
These bursts can travel vast distances before dissipating into giant radio-emitting lobes.
This process is pretty normal. Even the Milky Way has rays. 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 radio giant galaxies, and extreme models are especially key to understanding what their growth could be.
“If there are known hosts of galaxies that are an important cause of radio galaxy growth, then the largest group of giant radio galaxies is likely to possess them,” explained the researchers, led by astronomer Martijn Oei of the Leiden Observatory in the Netherlands. in his paper, which appeared in April of this year.
“Similarly, if there are particular large-scale environments that are highly conducive to the growth of radio galaxies, then the largest radio giant galaxies tend to reside in them.”
The team looking for these remains in data collected by the Low Frequency ARray (LOFAR) in Europe, an interferometric network consisting of around 20,000 radio antennas, distributed over 52 locations across Europe.
They promised information through a new pipeline, removing compact radio sources that could prevent detections of scattered lobes of radiation and correcting for optical distortion.
The resulting images, they say, represent the most sensitive searches ever performed for the galaxy’s radio lobes. Then they used the best recognition tool to locate their target: their eyes.
So a few billion light-years from the Milky Way galaxy they found Alcyone vomiting.
“We found what is in the projection the largest known framework made by a single galaxy – a radio giant galaxy with its own projected wavelengths [of] 4.99 ± 0.04 megaparsecs. They explained the true characteristic length of at least megaparsecs… 5.04 ± 0.05”.
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 fairly normal elliptical galaxy, embedded in the filament of the cosmic web, clocking in at about 240 million times the mass of the Sun, with a supermassive black hole at its center about 400 million times the mass of the Sun.
Both of these parameters are actually at the end for giant radio galaxies, which may account for some of the growth of the radio lobes.
“Besides the geometry, Alcyone and its host are suspiciously ordinary: the total low-frequency luminosity density, stellar mass and supermassive black hole mass are all lower than, although similar, the average of the radiating giant galaxies,” the researchers wrote.
“Thus, large galaxies or central black holes are not necessary to grow large giants and, if the observed state is representative of the source in its lifetime, neither is high radio power.”
It is possible that Alcyoneus sits in a region of space with a density lower than average, which could cause its expansion, or that interaction with the cosmic web plays a role in the growth of the object.
Whatever’s behind it, researchers believe Alcyone is still growing far out in the dark cosmos.
Research has been published Astronomy & Astrophysics.
An earlier version of this article was originally published in February 2022.
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