That's what he said

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.

Escaping the dark

We now return to those dark galaxies without dark matter. No, not those ones, the other ones. The originals.

This is a topic so controversial that even those claiming to have explained them say that the original discoveries were wrong, which is very confusing. But the best bet seems to be a tidal encounter. Since dark matter, like gas, is generally much more extended than the stars, it's easier to remove. Indeed, unlike gas, dark matter is collisionless, so if it happens to be in the centre of a galaxy on part of its orbit, there's nothing stopping it from moving out to greater distances a little later. Whereas the innermost gas gets trapped in the office forever, dark matter prefers a hybrid model : sometimes it's in the city centre, but sometimes it's in the vulnerable suburbs.

Topical metaphors aside, today's paper is the most convincing argument yet for a tidal origin. The authors re-examined an existing simulation that predates the whole hoo-hah, so this absolutely counts as a prediction. True, it would have been even better to predict these things ahead of the discovery itself, but there was no reason for anyone to look for them.

They find seven objects in their 21 Mpc-on-a-side simulation that lack dark matter. Most of them live in groups (three are all in the same group), and all are associated with a massive parent - more massive than the Milky Way. Now the properties of all but one are not all that similar to the notorious NGC 1052 DF2/DF4 (they have a few times as much stellar mass and, correspondingly, rather higher velocity diserpsions*), but this would probably be asking too much of a simulation. The main point about lacking dark matter is clearly satisfied, and they're not orders of magnitude different to the observational sample.

* These two parameters are intimately related though. The higher the stellar mass, the higher the velocity dispersion must be for the systems to be in stable equilibrium.

What I find most convincing about this is that they explain their objects as a rare but not exceptional process. Since there are at least two objects known in one group, and more dark matter free objects elsewhere, any explanation that relies on a freak encounter might have problems. It's a very tricky balance to find a mechanism that is routine enough that you actually expect to stand a reasonable chance of detecting the end results, but not so common that they should have been spotted everywhere and known about for decades.

The author's answer to this is that the satellites need to be very small in comparison to their parent galaxy (0.1% of its stellar mass) and come extraordinarily close - within the stellar body. And this seems to fit the bill quite nicely. One would expect most such encounters to result in the satellite merging with the cannibalistic parent or being utterly destroyed by it, but every so often one could just manage to survive. And another satisfying point is that while the after-effects of the encounter are still visible in some of their simulated objects, some show no visible signs of disruption at all : the main body of the galaxies survive long after the tidal tails have dispersed.

If anything, I worry that this explanation might overpredict the number of dark matter deficient satellites : around 30% of the most massive galaxies ought to have at least one such companion, they say. But such massive galaxies are themselves quite rare, and the smallest satellites are the hardest to measure, so this is not necessarily and outlandish fraction. Importantly, it's testable. We probably shouldn't get too hung up on the exact numbers given the limitations of observations and simulations alike - so long as it turns out these systems are not unique to NGC 1052, that's probably sufficient.

So this is very good stuff. What remains now is to look for analogues to those other galaxies without dark matter : the isolated, gas-dominated Ultra Diffuse Galaxies. Those, they say, might exist in their simulations as well... but that's another story.