The largest ever observed galaxy discovered in a giant cluster

Scientists have discovered the largest galaxy ever observed. Alcyoneus is located 3 billion light-years from Earth and is 5 megaparsecs in size. That is, it is a giant cluster of stars with a diameter of 16.3 million light years. For comparison purposes, the Milky Way, where the Sun is located, is about 100,000 light-years across. Until then, the largest observed galaxy was IC 1101 with an estimated size of 4 million light years. Thus, Alcyoneus is 163 times larger than our galaxy and 4 times larger than IC 1101.

The finding was published in a paper led by astronomer Martijn Oei in February and recently updated. Alcyoneus is located in the constellation Lynx and is a radio galaxy and its discovery shows how little we know about these giants. It hosts a supermassive black hole in its galactic core which exerts a gravitational pull on stars. It also emits colossal jets and lobes from its center.

lobes of and the growth of galaxies

Not all material that is pulled into an active black hole ends up at the event horizon. Some are thrown towards the poles instead of towards the accretion disks. Then this material is released into space in the form of plasma jets at relativistic speeds, very close to the speed of light. They can travel a huge distance before becoming radio-emitting lobes. Some of the host galaxies emit gigantic lobes, such as Alcyoneus, but scientists still don’t understand why this happens. Solving this could help explain why a radio galaxy grows to giant size.

“Similarly, if there are particular large-scale environments that are highly conducive to the growth of giant radio galaxies, the largest giant radio galaxies are likely to reside there.” explained the researchers of the Leiden Observatory in the article that discovered the giant galaxy.

The discovery of Alcyoneus

In their search for environments conducive to the growth of giant radio galaxies, they ended up finding Alcyoneus. They used the European Low Frequency ARay (LOFAR) radio telescope network. With the frequencies harvested, they removed compact radio sources that could interfere with diffuse radio lobe detections, and corrected optical distortion as well. With the analysis of the images obtained subsequently they were able to observe the galaxy.

“We have discovered what is, in projection, the largest known structure made from a single galaxy: a gigantic radio galaxy with an adequate projected length. [de] 4.99 ± 0.04 megaparsecs. The true correct length is at least… 5.04 ± 0.05 megaparsecs,” the astronomers explained.

Trying to understand it, they discovered that it is an elliptical galaxy, its stars do not move in an orderly rotation, as in spirals. It is embedded in a filament of the cosmic web and is about 240 billion times the mass of the Sun. Its black hole alone is 400 million times the mass of our star.

Despite this, astronomers found that their characteristics were quite common. “Geometry aside, Alcyoneus and its host are suspiciously common: the overall low-frequency luminosity density, stellar mass, and supermassive black hole mass are all lower than, though similar to, those of medial giant radio galaxies.” They further add “Thus, very massive galaxies or central black holes are not required for the growth of large giants, and if the observed state is representative of the source throughout its lifetime, neither is high radio power.”

They believe its unprecedented size may be related to where it is. A less dense region of space can favor the expansion of the galaxy. Another solution is that the cosmic web in which it is embedded plays an important role in its growth. Whatever the reason, astronomers believe it will grow even larger.

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Source: Olhar Digital