Earlier this year, astronomers found an absolute monster of a galaxy.
Hidden some 3 billion light-years away, Alcyoneus is a giant radio galaxy reaching 5 megaparsecs in space. It’s 16.3 million light-years long, and it is the largest known structure of galactic origin.
This discovery sheds light on our poor understanding of these colossi and what is driving their incredible growth.
But it could provide a path to a better understanding, not just of giant radio galaxies, but also of the intergalactic medium that drifts through the gaping voids of space.
Giant radio galaxies are yet another mystery in a universe full of mysteries. They consist of a host galaxy (i.e. the cluster of stars orbiting a galactic core containing a supermassive black hole), as well as colossal jets and lobes that shoot out from the galactic center .
These jets and lobes, in interaction with the intergalactic medium, act like a synchrotron to accelerate the electrons which produce a radio emission.
We’re pretty sure we know what’s producing the jets: an active supermassive black hole at the center of the galaxy. We call a black hole “active” when it swallows (or “accumulates”) matter from a giant disc of matter that surrounds it.
Not all accretion disk material swirling around in 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 towards the poles, where it is shot out into space in the form of jets of ionized plasma, at speeds representing a significant percentage of the speed of 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 understand is why in some galaxies they grow to absolutely gargantuan sizes, on the scale of megaparsecs. These are called giant radio galaxies, and the most extreme examples could be the key to understanding what drives their growth.
“If there are features of host galaxies that are an important cause of the growth of giant radio galaxies, then hosts of larger giant radio galaxies are likely to possess them,” explained the researchers, led by astronomer Martijn Oei de Leiden Observatory in the Netherlands. in their article, which was published in April this year.
“Similarly, if there are particular large-scale environments that are highly conducive to the growth of giant radio galaxies, then the largest giant radio galaxies are likely to reside there.”
The team looked for these outliers in data collected by the LOw Frequency ARray (LOFAR) in Europe, an interferometric network made up of around 20,000 radio antennas, spread over 52 sites across Europe.
They reprocessed the data through 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 conducted for radio galaxy lobes. Then they used the best pattern recognition tool available to locate their target: their own eyes.
That’s how they found Alcyoneus, spewing out of a galaxy a few billion light-years away.
“We have discovered what is in projection the largest known structure made by a single galaxy – a giant radio galaxy with an appropriate projected 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 found it to be a fairly normal elliptical galaxy, embedded in a filament of the cosmic web, clocked at about 240 billion 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 their lowest for giant radio galaxies, which could provide clues as to what drives radio lobe growth.
“Beyond the geometry, Alcyoneus and its host are oddly ordinary: the total density of low-frequency luminosity, stellar mass, and supermassive black hole mass are all lower, though similar, to those of mid-giant radio galaxies.” , the researchers wrote.
“Thus, very massive galaxies or central black holes are not necessary to grow large giants, and, while the observed state is representative of the source over its lifetime, high radio power is not. no more.”
It could be that Alcyoneus is sitting in a region of space with lower than average density, which could allow its expansion – or that interaction with the cosmic web plays a role in the object’s growth. .
Either way, however, researchers believe Alcyoneus continues to grow, far into the cosmic darkness.
The research has been published in Astronomy and astrophysics.
An earlier version of this article was first published in February 2022.
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