Astronomers have found a radio galaxy with a diameter 100 times as wide as our own Milky Way, yet the finders describe it as being “suspiciously ordinary” in every aspect other than its immense size.
The supermassive black holes at the heart of most galaxies spit out magnetized jets aligned with the rotation axis. Interaction of fast-moving charged particles with the intergalactic medium makes these jets bright at radio frequencies. Some jets spread out into lobes. A few jet/lobe combinations are exceptionally large (at least a Megaparsec across), earning the name Giant Radio Galaxies (GRGs) with no obvious explanation as to what makes them so big. In a quest to understand their nature, Leiden University PhD student Martijn Oei and colleagues went looking for the largest examples they could find.
As the authors reason in a paper accepted for Astronomy and Astrophysics but not yep peer-reviewed (preprint on ArXiv.org), “if there exist host galaxy characteristics that are an important cause for GRG growth, then the hosts of the largest GRGs are likely to possess them.”
They named the largest example they found Alcyoneus, after a giant enemy of Hercules whose name meant “mighty ass”. Alcyoneus is at least 5.04 Megaparsecs wide (which would make it 400,000 times the Kessel Run if Han Solo had understood parsecs are units of distance not time) and located around 3 billion light years away.
Yet when the authors tried to investigate what makes Alcyoneus so big they came up short. The galaxy that is producing these enormous ejecta is composed of stars with combined masses of 2.4 x 1011 times that of the Sun, about half that of the Milky Way. Its supermassive black hole has 400 million solar masses, or a hundred times Sagittarius A* at our own galaxy's heart, but is more than 100 times smaller than the largest known. If all it took to produce a giant radio galaxy was a black hole this big, there would be a lot more of them in the universe.
Besides its size, the one thing that stands out about Alcyoneus is that the pressure in its lobes is the lowest yet found, which, the authors state, makes it “the most promising radio galaxy yet to probe the warm–hot intergalactic medium.”
Estimates of GRGs' size tend to be imprecise – an object angled towards us will appear to be considerably smaller than it really is, so figures quoted are always minimums. Although more than a thousand GRGs are known, only ten are confirmed as longer than 3 megaparsecs, and just one, J1420-0545, approaches Alcyoneus in size.
The authors compare the features of Alcyoneus we can measure with those of other GRGs. If anything, its stellar and black hole mass are slightly on the low side, offering no clue as to why its radio jets are so enormous. Similarly, its luminosity density is fairly typical.
If galaxies' mass can't explain their GRG status, their surroundings provide an obvious next place to look. Alcyoneus doesn't appear to be part of a galaxy cluster, so the lack of interference from neighbors may be significant. However, the fact that five known galaxies are within 10 Megaparsecs “makes it implausible that Alcyoneus lies in a void,” the paper notes. Instead, the authors conclude, “Alcyoneus most probably inhabits a fillament of the Cosmic Web.”
Perhaps statistical analysis of larger GRGs will shed more light on the situation, but for the moment it remains a mystery as to how this galaxy could grow so big.