Astrophysicists earlier this year found a galaxy located about 3 billion light-years away, Alcyoneus is a giant radio galaxy 5 megaparsecs in space. At 16.3 million light-years across, it constitutes the largest known structure of galactic origin.
This discovery sheds light on our misunderstanding of these giants, and what drives their amazing growth, according to Science Alert, but it could provide a path to a better understanding, not only of the giant radio galaxies, but of the galactic medium that drifts in the vast voids of space.
Giant radio galaxies are another mystery in a world full of mysteries. It consists of a host galaxy (the group of stars orbiting the nucleus of a galaxy containing a supermassive black hole), as well as huge jets and lobes emanating from the center of the galaxy.
These jets and lobes interact with the intergalactic medium, acting as a synchrotron to accelerate electrons that produce radio emission.
And we sure know what’s producing the jets: an active supermassive black hole at the center of the galaxy. We refer to a black hole as “active” when it consumes (or “accumulates”) material from a giant disk of material around it.
Not all of the material in the accretion disk orbiting an active black hole ends up beyond the event horizon. A small part of it is somehow directed from the inner region of the accretion disk to the poles, where it is propelled into space in the form of jets of ionized plasma, at speeds a large proportion of the speed of light.
These jets can travel great distances before spreading out into giant lobes that emit radio waves.
This process is very normal. Even the Milky Way has radio lobes. What we don’t really have a good idea of is why, in some galaxies, they grow to extremely massive sizes, on megafarsec scales. These are called giant radio galaxies, and the most extreme examples can be key to understanding the drivers of their growth.
“If there are characteristics of host galaxies that are an important reason for the growth of giant radio galaxies, they are likely to be possessed by the hosts of the largest giant radio galaxies,” the researchers, led by astronomer Martin Owe of the Leiden Observatory in the Netherlands, explained. In their paper published in April this year.
Likewise, if there are certain large-scale environments that are significantly conducive to the growth of giant radio galaxies, then the largest giant radio galaxies are most likely to be found in them.”
The team went on to look for these outliers in data collected by Europe’s LOw Frequency ARray (LOFAR), an interferometric network of about 20,000 radio antennas, distributed at 52 sites across Europe.
They reprocessed the data through a new pipeline, removing embedded radio sources that might interfere with detections of diffuse radio lobes, and correcting for optical distortion.
They say the resulting images represent the most sensitive search ever conducted for galactic lobes. Next, they used the best pattern-recognition tool available to locate their target: their eyes.
And that’s how they found Alcyoneus, which is emanating from a galaxy a few billion light-years away.
“We have detected what is in projection the largest known structure created by a single galaxy – a giant radio galaxy of suitable length expected [بـ] 4.99 ± 0.04 megaparsecs. The true proper length is at least … 5.04 ± 0.05 megaparsecs,” they explain.
Once the lobes were measured, the researchers used the Sloan Digital Sky Survey to try to understand the host galaxy.
They found that it is a fairly ordinary elliptical galaxy, embedded in filaments of the cosmic web, with a mass of 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.
And both of these parameters are actually at a lower limit for giant radio galaxies, which could provide some clues as to what drives the growth of radio lobes.
“Geometry aside, Alcyoneus and its host are suspiciously ordinary: the total low-frequency luminosity intensity, stellar mass, and supermassive black hole mass are less than those of the medial giant radio galaxies, despite their similarity,” the researchers write. Very massive galaxies or central black holes are not necessary for the development of large giants, and if the observed condition represents the source throughout its lifetime, then there is also no high radio energy.”
And Alcyoneus could be sitting in a region of space that is less dense than average, which could enable its expansion — or that interaction with the cosmic web plays a role in the object’s growth.
Whatever is behind it, researchers believe Alcyoneus is still growing larger, farther out in the cosmic darkness.
The research has been published in the journal Astronomy & Astrophysics.