Largest galaxy discovered: it shouldn’t actually exist

Deepak Gupta February 17, 2022
Updated 2022/02/17 at 7:32 AM

Humans have been studying the stars for thousands of years. But only in the last few centuries have we begun to understand more and more. Especially over the past few decades, research has encountered new objects that claim their attention. She brought new moons, planets, stars and entire galaxies together with the black holes hidden within them into our consciousness. Now astronomers have made a new breakthrough: They have them largest galaxy discovered that is known so far.

Largest galaxy named after ancient legend

This gigantic formation of countless celestial bodies is located at a distance of about three billion light years from our Milky Way. It’s a radio galaxy an unprecedented five megaparsecs (Mpc) across—roughly 16.3 million light-years across. For comparison: The Milky Way has a diameter of just 105,700 light years (about 0.03 parsecs). This makes the largest galaxy around 154 times larger than ours.

It was not for nothing that its discoverers gave it the name Alcyoneus (German: Alkyoneus). This Greek legendary figure was born as the son of Gaia and Uranus. Alkyoneus is considered the oldest giant after his brother Pindar. And the star cluster that now bears his name is truly gigantic.

Martijn Oei from the Leiden Observatory in the Netherlands and his team published theirs in the journal Astronomy & Astrophysics (via and sometimes explain some of the peculiarities associated with the galaxy. Because Alcyoneus is not just any random collection of celestial bodies, but a radio galaxy. In other words, it emits extremely strong continuum synchrotron radiation that can be observed in the radio wave range. Their sphere of influence sometimes extends well beyond their visible regions, due in part to the active supermassive black hole at their core.

Fund raises new questions

As part of their study, the scientists point out that almost a thousand radio galaxies are known. Only about 100 have so far reached a size of more than two Mpc. Only ten of them are larger than three Mpc. The largest known specimen to date had a diameter of approximately 4.9 Mpc. With the discovery of Alcyoneus, the team not only encounters a new record holder. Because the now largest galaxy has a feature that shakes the current state of research.

This specimen is an elliptical galaxy whose galactic center, the black hole Alcyoneus*, has about 400 million times the mass of the sun. Sigittarius A*, the center of the Milky Way, has about 3.7 million solar masses. According to the current state of research, it should not even exist. Until now, it has been assumed that much larger black holes are needed to bind galaxies of such dimensions. The astronomers conclude:

“Very massive galaxies or central black holes are not necessary to produce large giants, and if the observed state is representative of the source over its lifetime, high radio power is not necessary either.”

M.Oei et al. (via

This means that the team is faced with new questions. First and foremost: what is necessary to create a structure like the largest galaxy?

First hypotheses

One of the researchers’ considerations is that Alcyoneus is moving in a region of space that has a low density. This allows for far greater expansion. On the other hand, it is possible that the galaxy’s interaction with the cosmic web plays a role in its growth. This is an almost endless network of gigantic thread-like gas structures that run through the entire universe.

Whatever hypothesis may come true, it will take some research to fully understand Alcyoneus and the reasons for the size of the star cluster.

Source: “The discovery of a radio galaxy of at least 5 Mpc” (2022; Astronomy & Astrophysics)

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