Astronomers discover the largest galaxy ever, and the scale will break your brain: ScienceAlert

Astronomers discover the largest galaxy ever, and the scale will break your brain: ScienceAlert

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

Alcyoneus, some 3 billion light-years away, is a giant radio galaxy reaching 5 megaparsecs into space. That’s 16.3 million light years long, and it forms the largest known structure of galactic origin.

The discovery highlights our flawed 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 yet another mystery in a universe full of mysteries. They consist of a host galaxy (that is, the star cluster that orbits a galactic core that has a supermassive black hole), as well as colossal jets and lobes erupting from the galactic center.

These jets and lobes, interacting with the intergalactic medium, act as one synchrotron to accelerate electrons that produce radio waves.

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 swallows (or “attracts”) material from a giant disk of material around it.

Not all of the material in the accretion disk swirling in an active black hole inevitably ends up beyond the event horizon. A small portion of it is somehow directed from the inner part of the accretion disk to the poles, where it is blown into space in the form of jets of ionized plasma, at velocities a significant percentage of the speed of light.

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

Halcyon galaxy radio lobes
The radiolobes of Alcyoneus. (Oei et al., Astronomy & Astrophysics, 2022)

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 gigantic sizes, on megaparsec scales. These are called giant radio galaxies, and the most extreme examples could be the key to understanding what drives their growth.

“If host galaxy properties exist that are an important driver of the growth of giant radio galaxies, then the hosts of the largest giant radio galaxies are likely to possess them,” said the researchers, led by astronomer Martijn Oei of the Leiden Observatory in the Netherlands. . explained in their paper, which appeared in April of this year.

“Similarly, if certain large-scale environments exist that are very conducive to the growth of giant radio galaxies, then the largest giant radio galaxies are likely to be located there.”

The team set out to find these outliers in data collected by the LOW Frequency ARray (PROMISES) in Europe, an interferometric network consisting of about 20,000 radio antennas, spread over 52 locations across Europe.

They reprocessed the data through a new pipeline, removing dense radio sources that could interfere with the detection of diffuse radiolobes, 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 for locating 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 created by a single galaxy – a giant radio galaxy with a projected correct length [of] 4.99 ± 0.04 megaparsec. The real 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 fairly normal elliptical galaxy embedded in a filament of the cosmic webclocking in 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 parameters are actually on the low side for giant radio galaxies, which could provide some clues as to what drives radiolobe growth.

“Geometry aside, Alcyoneus and its host are suspiciously common: the total low-frequency luminosity density, stellar mass, and supermassive black hole mass are all lower than, though similar to, those of the medial giant radio galaxies,” the researchers wrote.

“So very massive galaxies or central black holes are not necessary to grow large giants, and if the observed condition is representative of the source over its lifetime, then high radio power is not.”

It could be that Alcyoneus is in a region of space with a lower density than average, which allows for its expansion – or that interaction with the cosmic web plays a role in the object’s growth.

Whatever’s behind it, the researchers believe Alcyoneus is still getting bigger, far away in the cosmic dark.

The research has been published in Astronomy & Astrophysics.

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

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