Discovery of the largest galaxy ever observed
The discovery highlights our poor understanding of these giants 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 drifting into the yawning voids of space. Giant radio galaxies are another mystery in a universe full of mysteries. They consist of a host galaxy (which is the cluster of stars orbiting a galactic core containing a supermassive black hole), as well as colossal jets that erupt from the galactic center.
These jets, interacting with the intergalactic medium, act as a synchrotron to accelerate the electrons that produce radio emissions. We're pretty sure we know what the jets produce: an active supermassive black hole at the galactic center. We refer to a black hole as "active" when it is devouring (or "accrescendo") material from a gigantic disk of material around it.
Not all the material in the accretion disk swirling into a black hole active inevitably ends beyond the event horizon. A small fraction of it is somehow channeled from the inner region of the accretion disk to the poles, where it is blasted into space in the form of ionized plasma jets, at speeds a significant percentage of the speed of light. These jets can travel enormous distances before spreading into gigantic radio-emitting lobes.
This process is quite normal. The Milky Way also has radio lobes. What we don't really have a good grasp of is because, 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 there are features of the host galaxy that are an important cause for the growth of the giant radio galaxy, then it is likely that the hosts of the largest giant radio galaxies possess them, ”explain the researchers, led by astronomer Martijn Oei of the Leiden Observatory in the Netherlands, in their pre-press document, which was accepted for publication in Astronomy & Astrophysics. "Likewise, 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 in them."
Credits: Hubble NASA / ESA The team searched for these outliers in data collected by the LOw Frequency ARray (LOFAR) in Europe, an interferometric network consisting of approximately 20,000 radio antennas, spread across 52 locations across Europe. They reworked the data through a new pipeline, removing compact radio sources that could interfere with scattered radio lobe detections and correcting for optical distortion.
The resulting images, they say, represent the most sensitive research ever conducted for the lobes of radio galaxies. Then, they used the best pattern recognition tool available to locate their target: their own eyes. This is how they found Alcyoneus, which ejects material from a galaxy a few billion light years away.
"We have discovered what is projected into the largest known structure made from a single galaxy, a giant radio galaxy with a correct projected length [of] 4.99 ± 0.04 megaparsec. The true correct length is at least… 5.04 ± 0.05 megaparsecs, ”they write. 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 normal elliptical galaxy, embedded in a strand of the cosmic web, which it reaches 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 in the low-end range for giant radio galaxies, which could provide some clues as to what drives the growth of radio lobes.
"Beyond geometry, Alcyoneus and his host are suspiciously ordinary: the total density of low-frequency luminosity, the stellar mass and the mass of the black hole supermassive are all inferior, albeit similar, to those of giant medial radio galaxies, ”the researchers write. "Therefore, very massive galaxies or central black holes are not necessary for large giants to grow and, if the observed state is representative of the source throughout its life, even high radio power is not such".
It could be that Alcyoneus is sitting in a region of space with a lower than average density, which could allow for its expansion, or that interaction with the cosmic web plays a role in the growth of the object. Whatever is behind it, however, researchers believe Alcyoneus is continuing to grow even larger, far away in the cosmic darkness.
