Or something. I mean, there just must be a dozen brilliant innuendos regarding any claim for the largest radio galaxy in the Universe. There just must be.
That's the topic of today's paper. Low frequencies aren't my area of expertise (then again, neither are high frequencies...) but these very low energy emissions are another good way to trace out gaseous structures not visible at other wavelengths. In this case, the aptly-named giant radio galaxies. These are - wait for it - galaxies... which emit at radio wavelengths... and are gigantic.
Ahem.
Anyway, such objects generally seem to arise from supermassive black holes. If you haven't come across radio galaxies before, you've almost certainly seen spectacular images of enormous jets emanating to enormous distances from the cores of comparatively tiny host galaxies. You bloody well ought to, at any rate, since I went to all that trouble to show just how gigantic these features can be. This one, though, is even bigger : a whopping 5 Mpc (15 million light years and change).
I'm probably going to have to update the size chart for that one.
Not being expert in this area, the first question that came to my mind was : are you sure you're measuring it right ? You didn't, say, just take a close-up picture to make it look bigger, did you ? DID YOU ?
No, they didn't. Unlike a lot of spectacular discoveries, it turns out giant radio galaxies aren't all that uncommon. Over a thousand are known, of which a hundred are longer than 2 Mpc and ten exceed 3 Mpc. One reaches to 4.9 Mpc. So finding one which is a bit longer again is very cool, but not unprecedented. It's like finding the largest elephant, not like discovering elephants for the first time.
The second question that came to mind was : how did it get so big ? Did it see a pretty galaxy and it just couldn't control itself ?
Look, the world is still in the midst of a pandemic but seems determined to ignore it, the UK government can't bring itself to tell the truth about a piece of cake, and Russia thinks the year is closer to 1822 than 2022. If ever there was a need for really, really stupid jokes, it's now.
Somewhat surprisingly, there's nothing particularly special about the optical galaxy. Neither the galaxy itself nor its supermassive black are exceptionally large - indeed the authors describe them as "suspiciously ordinary". So it's not an outrageous level of power being generated. More promising seems to be the environment. Although it's not in a full-on void, it's not in a cluster either - it's just in a large-scale galaxy filament, like most other galaxies. Filaments do have their own gas, but at very low density. The most likely explanation for Alcyoneus' enormousness, then, is probably that there's just not much around to stop it.
Alcyoneus ? Yes. Quoth the authors :
Alcyoneus was the son of Ouranos, the Greek primordial god of the sky. According to Ps.-Apollodorus, he was also one of the greatest of the Gigantes (Giants), and a challenger to Heracles during the Gigantomachy — the battle between the Giants and the Olympian gods for supremacy over the Cosmos. The poet Pindar described him as ‘huge as a mountain’, fighting by hurling rocks at his foes.
Seems appropriate to me.
And there's also scientific value, beyond the obvious cool factor of finding anything superlative. The density of gas in the filaments is hard to constrain because it's very thin, but it might well be important in galaxy evolution. In clusters, the ram pressure from a galaxy moving through the external gas can be strong enough to rapidly remove even the densest atomic gas, but a similar effect might be at work in filaments too. Here the surrounding gas probably wouldn't be dense enough to remove the star-forming material, but it could potentially remove the more extended gaseous halos - the reservoirs of fuel for future star formation.
The giant radio lobes of Alcyoneus are a way of testing this. Being at such low densities, they might be approaching pressure equilibrium with the surrounding material, allowing at least in principle a way to estimate the density of the thin filamentary gas itself. So far as I can tell, the author's don't actually do this - seems like something that needs additional work - but they say that Alcyoneus "represents the most promising intergalactic barometer of its kind yet". Well, if nothing else, kudos to them for the lively metaphors. And not an innuendo in sight.
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