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Tuesday, 10 November 2015

Keenan's Ring

Or, Galaxies Behaving Badly...

I've been itching to write this post for a while. They say the first one's always the hardest, and that's certainly true of astrophysical papers. Much toil has gone into this over the last year or so on the paper alone, but the results are not only very interesting, they're also pretty.

If we could see the hydrogen gas in the galaxy M33, it would appear around five times larger than the full Moon.
The super-short version is that we've found a new hydrogen cloud (not shown above, you'll have to keep reading for that) near to the Triangulum galaxy. It's the largest, most massive cloud that's been found there in over 30 years, and it's ring shaped. We have no idea why it's a ring, let alone what the cloud actually is. A giant starless galaxy ? A huge shell of hydrogen somehow thrown out of the galaxy ? Nothing seems to work. But to understand why this is so difficult to explain, read on.

(Note that I could have given this the clickbaiting title of "Arecibo Scientists Baffled By Giant Hydrogen Cloud" or, "Find Out Why Astronomers Can't Explain This Huge Celestial Ring" or even, "One Young Scientist Went Looking For Galaxies And You Won't Believe What Happened Next". But I didn't, because I am not a moron.)

As usual, the first part is an introduction. If you'd rather skip straight to the new results, scroll down to the "Hydrogen Hunting" section.

Trouble in the Neighbourhood

At first glance, our Local Group of galaxies is a somewhat dull place. Our Milky Way is like a great galactic introvert, hanging out with just a couple of close friends (Andromeda and Triangulum - also known as M31 and M33) and steering clear of heaving nightclubs like the Virgo cluster. Or so you might think.

The Local Group (left) compared to the nearest rich galaxy cluster (Virgo, right). The field of view - about 6 million light years - is the same in each case; the galaxy sizes have been exaggerated by a factor of twenty. Galaxy images come from the Sloan Digital Sky Survey. Small galaxies in the Local Group are shown as faint transparent fuzzy patches. You can just about see Andromeda flying past, but good luck spotting the tiny Triangulum.
But look a little closer and the Local Group turns out to be less of a fancy soiree and more of a rowdy house party. True, there are only three giant galaxies, whereas the Virgo cluster has many hundreds (possibly thousands). If you count smaller (dwarf) galaxies, the Local Group still only has around a hundred at most - it's nowhere near as crowded as a cluster, but we haven't got to the good stuff yet.

I've already described how the numbers and orbits of those dwarfs aren't anything like what we expected (have a look here, points 5 and 6). In short there aren't anywhere near as many dwarfs as models predict, and while they should be buzzing around the giant galaxies like a swarm of bees, they're actually orbiting in loose planes at right-angles to the discs.

So the Local Group is maybe like some sort of non-conformist hipster club where no-one tells anyone what to do. But that's just what we see in the visible light. When we look at the atomic hydrogen gas* using radio telescopes, things get even stranger - and messier.

* See link for details, but remember that atomic hydrogen can only be seen using radio telescopes - so colours are false.

Two major features dominate the group. The first, and most spectacular, is the Magellanic Stream shown above - a huge stream of hydrogen gas that runs halfway around the sky. Possibly more. Whenever observations get more sensitive, it seems we find it's even longer than we realised.

Most - possibly all - of this gas seems to have come from the Magellanic Clouds, two nearby dwarf galaxies. Exactly how the gas got into this peculiar configuration is not well understood, but probably has something to do with how the two galaxies are interacting with each other whilst orbiting the Milky Way. The stream isn't spectacular just because it's close though. At well over half a million light years, it's one of the longest hydrogen streams known.

Fading between an optical image of the Magellanic Clouds and the hydrogen data which shows the gas stream.
The second feature is another, much fainter stream linking Andromeda and Triangulum. This is another dramatic structure at 850,000 light years long. It's also one of the faintest hydrogen streams known, and was only detected by a survey of exceptionally high sensitivity and low resolution. So, unfortunately, there aren't any pretty pictures of this one.

Map of the M31-M33 stream from a paper by Braun & Thilker 2004.
So whereas the Local Group has three large galaxies and two hydrogen streams which are nothing less than awesome in extent, the Virgo cluster may have thousands of galaxies but it's only got four major streams. Looks like it's a win for the hipster house party, then. Of course the Virgo cluster has its own weirdness, but that's another story.

To understand the new results, there's one other important feature of the Local Group you need to know about. Although the main part of the Magellanic Stream doesn't go anywhere near Triangulum, a bunch of smaller clouds do. Could there be some connection between the Magellanic and the Andromeda streams ? I dunno. M33 and M31 are a lot further away than the Magellanic Clouds and the stream would have a really weird, sharp kink in it, so it could just be a complete coincidence.

Probably the best map of the stream produced to date, from a paper by Nidever et al. 2010. M33 is just above Wright's Cloud (which we'll talk about soon).

Hydrogen Hunting

Now that we've got the proper context, the new results. Most of the smaller galaxies in the Local Group are orbiting the Milky Way and Andromeda. Only one is believed to be associated with Triangulum, and that's not certain. Together with all these crazy streams (especially the M33-M31 stream), back in 2006 it seemed like a good idea to look at the hydrogen in and around M33 as part of the Arecibo Galaxy Environment Survey, AGES. This is one of the most sensitive atomic hydrogen surveys ever performed, so if there's anything interesting, AGES is what will find it.

"If all else fails, point a 300m telescope at it."
It took us five years to complete the observations - without any guarantee that we'd detect anything interesting in the end. That's the problem with a telescope that can't be steered much, but it isn't normally this bad. The difficulty was that the survey area is at the absolute northern limit of where Arecibo can see, so often we could only observe it for an hour a night. Factor in that other projects sometimes get priority, the fact that we can't see the region at all for more than half the year, and various equipment failures, and five years doesn't seem quite so bad.

Five years of stating at computer screens...
More staring at screens...
Everybody look serious now. Come on people, this is science.
Occasional bouts of madness began to set in...
... which were usually solved with late-night Dominion sessions.
Sometimes even the ALFA receiver went a little nuts and had to be taken down for repairs.
Most of the people above didn't do observations of M33. In fact most of the time we just did it by setting up a script and going home.

After all this, what did we find ? Well, firstly that the hydrogen of Triangulum is considerably more extended than what the last major survey of the area had revealed. Some of the "clouds" that ALFALFA had found turned out to just be slightly denser parts of this larger disc. Also, the disc is really quite faint - so faint, according to earlier work, it should be ionized. But it isn't. You could call it, "The Hydrogen Cloud So Large It Shouldn't Exist", if you want to clickbait it*.

* But don't. It's not the only hydrogen detection known below the ionization threshold nor the first such discovery.

Animation showing the hydrogen in M33 at different sensitivity levels. If your sensitivity isn't very good, all you'll see is the densest gas in the stellar disc. Go a bit deeper and you'll see things are a lot more extended, and part of the disc (upper right) is warped. Go as deep as AGES and you'll see that even this extended component is actually just part of something even larger.
There's one more brief digression I must make for the sake of non-regular readers. Hydrogen observations depend on frequency - that is, how fast something is moving towards or away from us. Which means our radio maps are actually 3D data cubes. They're very cool to look at (and extremely useful, as we shall see), but velocity is not the same as distance. For example some of the hydrogen in M33 has the same velocity away from us as does the hydrogen in parts of the Milky Way. So in these data cubes it can look as though the hydrogen in M33 and the Milky Way overlap each other - but in reality they're actually separated by over a million light years.

M33 is the big bright thing in the middle. Some hydrogen from the Milky Way can be seen as the big flat thing at one edge of the cube.

Anyway, although the new observations showed that some clouds weren't clouds at all, they also found new ones that pretty much exactly balanced out the numbers. Just enough to agree with how many satellite galaxies models of galaxy evolution predict there should be... but none of them have any stars !

Could these be the "missing satellites" that people have been searching for for so many years ? It's not impossible, but I doubt it. Firstly, it doesn't seem very likely that only this one galaxy happens to have the right number of companions and they just happen to be made of gas rather than stars - the model's so badly broken for other galaxies, fixing it with this one wouldn't really help. Secondly, the motions of gas in the clouds don't match the model predictions*. Thirdly, they're not distributed like the predicted "swarm of bees" at all - they're found in a distinct band. There's a hint that some of these clouds are actually linked together and part of a much larger structure.

* This is a debatable point though. More precisely, models predict how fast the gas should be moving within galaxies, and also how it's moving - that is, they predict it should be rotating. The observations say the motions are random. But a new paper has just been submitted saying that maybe the motions should be random after all.

Some of the clouds we detected with AGES (M33 is right in the middle, but at a different velocity so it's not visible here). Notice that there aren't any clouds in the top half of the survey area. Though it is awfully strange that the number of clouds is almost exactly enough to account for the missing satellites.

And two of the clouds are much, much bigger than the others. One of them - Wright's Cloud, which we mentioned earlier - has been known about since 1979. The most popular explanation is that it's part of the Magellanic Stream, or at least related to it in some way. Although it's not that close to the main stream, it does seem to follow those smaller clouds heading from the stream towards M33. It's a bit strange though, since Wright's Cloud is far larger than any other cloud at that distance from the Magellanic Clouds.

Now, 36 years later, we can reveal another giant cloud right next to Wright's Cloud that no-one had ever detected before. Well, that's not quite true. Parts of this new cloud had been found by ALFALFA. But our new observations were more sensitive over a larger area, and they've revealed that these clouds are actually just a small part of a much larger ring :

The centre of M33 (not shown) is just above and to the right of the ring.
We're calling this Keenan's Ring after "Princess" Olivia Keenan who discovered it and did most of the hard work of writing the paper. There isn't really any convention for naming hydrogen clouds, but if we get this in common usage it will probably stick. After all, it sounds a lot better than its catalogue designation AGESM33-31.

Map of all the clouds in the area. Keenan's Ring is at about as large as M33 and yet no-one knew about it until now.
So what the heck is it ? It all depends on how far away the cloud is, which we can't measure directly. If it's as far away as M33, then the thing is about 60,000 light years across - as big as the hydrogen of M33 itself ! And that would be seriously strange, because there's no obvious way to form a stonkin' great ring of hydrogen gas like this. Models show that you can do it through colliding galaxies, but M33 doesn't look as though it's had any kind of major collision recently. The gas in the Ring appears to be completely separate to the gas in M33.

The full data cube. Colours are arbitrary - blue was chosen to highlight the gas in the Milky Way, which fills the entire field of view. Orange shows everything else. M33 is the big orange blob in the middle. Wright's Cloud is the brightest-looking orange cloud which is cut off by the edge of the survey area. Keenan's Ring is visible, as are a bunch of other smaller clouds. The data looks noisier around the Ring and the Cloud, but this is just an artifact due to how the data was visualised.

Because this gas cloud is so dang large, could it be a giant, failed galaxy ? A tempting idea. Although M33 doesn't seem to have collided with anything recently, there is a strange "warp" in its hydrogen disc, and Keenan's Ring is on exactly the opposite side of the galaxy. Which is what you might expect if a giant object had come sailing past. Also, the densest gas in the Ring is closest to M33, suggesting that M33 is exerting a gravitational influence on it (which isn't the case for Wright's Cloud, which has a higher density in its center).

But this idea doesn't really work. Although there is a small velocity gradient (the gas on one side is moving at a different speed to that on the other) across the Ring, which is often a sign of rotation, it's far smaller than for M33 (~30 km/s compared to ~180 km/s). A galaxy as large as M33 really ought to be rotating as fast as M33 - if it isn't, then it isn't as massive, so it's far less likely to explain the warp. And if it was massive, that would make the predictions of the models of how many galaxies there are even worse, not better.

It also doesn't seem a likely coincidence that Keenan's Ring and Wright's Cloud are at almost exactly the same velocity. If Wright's Cloud is really just another part of the Magellanic Stream, then it seems probable that Keenan's Ring is as well. This, in my opinion, is the most likely explanation. But it's not without major difficulties either. Why in the world should there be two massive clouds near the end but significantly offset from the main stream ? And if it's not a coincidence that Wight's Cloud is part of the Magellanic Stream because it's so close, then surely by the same token it's at least equally likely to be associated with M33 in some way as well ?

No-one seems to have any answer to this. And it's also worth remembering that Wright's Cloud and Keenan's Ring are at different velocities to the other clouds detected in this area, suggesting that they might have different origins. Yet they're also very different to each other. Wright's Cloud is larger, much more massive, doesn't show any signs of a velocity gradient, has an irregular structure, and is denser in the centre. Keenan's Ring does have a velocity gradient (albeit a small one), is much denser on one side than the other, and is of course ring shaped.

And perhaps the most difficult question to answer applies to any scenario : why is it a ring ? A disc, well, that's fine - could be a giant galaxy, or just a cloud, whatever. An amorphous blob like Wright's Cloud - yeah, also possible, could be interacting with all the other clouds and M33, no problem. But there's no obvious reason why the gas in the middle should be missing. Where's it gone ?

Could it just be that the "ring" is a bunch of clouds which happen to line up and look like a ring ? Not likely. That would require an extremely unlikely chance alignment of clouds - there's no reason you'd expect the central region to be underpopulated just by placing clouds at random. And although the velocity gradient is small, it does have one. If it was a bunch of random clouds, you'd expect the velocity of each cloud to be different. But the velocity of the gas across the Ring varies quite smoothly from one side to the other, which you wouldn't expect if it was made of separate clouds. It's possible, but not credible.

Map showing the velocity of the gas at each point in the Ring

Honestly we really just don't know what this is. Supernovae explosion are known to blast holes in the hydrogen, but if it's close to M33 then the hole is about ten times larger than any other known holes. Nor are there any obvious star clusters (massive stars aren't thought to be able to form in isolation), so if it's inside our own galaxy the Ring would be much smaller but probably just as strange. And it would be one heck of a coincidence if this structure just happened by chance to not only be so close on the sky to Wright's Cloud but at such a similar velocity.

Like all the most interesting discoveries, this one poses a lot more questions than it answers. Which, if you ask me, makes it five years well spent.

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