Despite having a diameter 100 times that of our own Milky Way, astronomers have discovered a radio galaxy that is “suspiciously ordinary” in every way except its enormous size. Most galaxies’ supermassive black holes spit out magnetic jets that are aligned with the rotation axis. These radio-frequency jets are caused by the interaction of fast-moving charged particles with the intergalactic medium. Some of the jets split apart into lobes. A few jet/lobe combinations are unusually huge (at least a Megaparsec wide), earning them the moniker Giant Radio Galaxies (GRGs), despite the fact that there is no evident explanation for why they are so large.
Martijn Oei, a PhD student at Leiden University, and his colleagues began on a search for the largest specimens they could locate in order to better understand their nature. “If there are host galaxy properties that are an important cause for GRG expansion, then the hosts of the greatest GRGs are likely to contain them,” the author’s reason in an article accepted for Astronomy and Astrophysics but not yet peer-reviewed (preprint on ArXiv.org).
They dubbed the largest specimen Alcyoneus, after Hercules’ enormous foe whose name meant “great ass.” Alcyoneus is around 3 billion light years away and is at least 5.04 Megaparsecs wide (which would make it 400,000 times the Kessel Run if Han Solo had realized parsecs are units of distance, not time). However, the authors were unable to determine what makes Alcyoneus so large. The galaxy ejecting these massive ejecta is made up of stars with a combined mass of 2.4 x 1011 times that of the Sun, or almost half that of the Milky Way.
Its supermassive black hole has 400 million solar masses, or a hundred times the mass of Sagittarius A* at the center of our galaxy, although it is more than a hundred times smaller than the largest known. There would be a lot more huge radio galaxies in the cosmos if all it took to create one was a black hole this massive.
Apart from its size, Alcyoneus is notable for having the lowest pressure in its lobes ever discovered, making it “the most promising radio galaxy yet to investigate the warm–hot intergalactic medium,” according to the astronomers. GRG size estimates are notoriously imprecise — an object pointed towards us appears much smaller than it actually is, therefore measurements cited are always minimums. Despite the fact that there are over a thousand GRGs known, only 10 have been confirmed to be longer than 3 megaparsecs, and only one, J1420-0545, is close to Alcyoneus in size.
The authors compare the characteristics of Alcyoneus with those of other GRGs that can be measured. It’s stellar and black hole masses are on the low side, providing no explanation for why its radio jets are so massive. Its luminosity density is also quite common. If the mass of galaxies isn’t enough to explain their GRG status, the next apparent place to investigate is their surrounds. Because Alcyoneus does not appear to be part of a galaxy cluster, the lack of nearby interference could be significant.
The fact that five known galaxies are within 10 Megaparsecs of Alcyoneus, however, “makes it implausible that Alcyoneus lies in a void,” according to the paper. “Alcyoneus most likely inhabits a fillament of the Cosmic Web,” the authors conclude. Perhaps statistical examination of larger GRGs will throw additional light on the situation, but how this galaxy grew so huge remains a mystery for the time being.
