The End Of Darkness

My goodness me, another paper ! What is this, a science blog or something ?

This one is by my long-term partner in crime, the inestimable Robert Minchin. After the last few papers – one on a single weird galaxy and two on source extraction – this is a return to my favourite topic : dark galaxies.

And not just any dark galaxies...

Well, the joke works if you're British, at any rate.

What I meant to say is that these are the objects I've spent most of my career on. If you haven't read the link, a dark galaxy is just a galaxy which has some gas, maybe a very few stars (but preferably none at all), all embedded in the classic dark matter halo that we all know and love : the very thing that distinguishes galaxies from all the other crap floating around in the majesty of the cosmos.

Look, I've been through this dozens of times. I'm not gonna do it again. Go and read the link, I'll wait.

Go ! Read up and report back. Off you pop.

Okay, you ought to bloody well know about these things in general by now. But I'll forgive you if you can't quite remember why I like these particular candidates so much. Mainly because I'm extremely biased and narcissistic... that is, they turned up in the data I was analysing during my PhD, and were the main highlight of my very first papers*. I've been working on these for the best part of two decades, which is a scary thought if nothing else.

* It's all "me, me, me" around here.

Yes, yes, but apart from keeping me employed, what's so fascinating about them ? No-one's going to keep paying anyone to study genuinely boring objects, after all.

Well, it's a combination of two factors. The first is that they have a high line width. That is, we can see that parts of them are moving at very different speeds along our line of sight (towards or away from us). This is exactly what we see with normal, rotating galaxies, and it's normally a pretty good signature that they have a lot of dark matter. 

But this, we've learned, isn't decisive. We didn't have good spatial resolution of the clouds, so we couldn't be sure that we were really seeing rotation. Simulations have shown that when galaxies interact with each other, the debris that gets torn off can (briefly) exactly mimic this apparent signature of rotation, so it was possible we were just being fooled. And without rotation, there's no reason to think there's any dark matter.

Really good data would let us map the motion very precisely, like on the left. Here we can see that one side is moving differently to the other, with a smooth gradient across the object : a classic signature of stable spin. But for these objects, all we have is something like the spectrum on the right. The width of the bump looks like rotation, but without an actual map, we can't be sure. (This example was taken from this paper)

That's where the second parameter comes in : these objects are relatively isolated. Our simulations have shown that when fake dark galaxies are formed, they ought to be accompanied by massive streams of material from their parent galaxies. There's no sign of any that with these guys. In fact, their high line width makes this all the more surprising. The simulations show that features with widths this high (180 km/s) ought to be the fastest-moving parts of the stream of all, and therefore the parts that disperse the most rapidly. 

So the lack of a stream, if these are fakes, is paradoxical : how can the fastest-dispersing part of the cloud be the longest-lived ? It all makes a lot more sense if we're seeing genuine rotation due to dark matter, since rotation can be stable on indefinite timescales.

And there's more. We found eight such clouds in a relatively small part of the Virgo Cluster, almost 10% of all the gas detections in that same area. Now if these were transient, unstable objects, they'd have to be forming at a very high rate indeed for us to detect them. To find one or two might be plausible*, but eight ? Nah, that's silly.

* Even then, only just. Actually the simulations found that we could essentially never produce any isolated clouds with line widths this high, because they disperse so bloody quickly.

The heart of the problem is simple. To find an HI stream with some weird velocity structure that briefly looks like a dark galaxy is quite possible. But to find them in isolation, with the rest of the stream having gone but the fastest-dispersing bit somehow still surviving... that flat-out doesn't work. Not for this many objects, at any rate.

---

What we've been lacking for all these years are two things : better statistics and better resolution. For statistics, you'll have to wait for my PhD student's first paper. Today we're dealing with resolution.

Yes, today. Yes, that's nine years after applying for time on the VLA. Stuff kept coming up, mmmkay ?

Seriously, it did. I didn't know much about how to reduce the data and I had a lot more low-hanging fruit to pick. Plus there was that whole global pandemic thing, during which I recoded 20,000 lines of Python code, amongst other things.

Step forward Robert. Having previously been head of astronomy at Arecibo (which collapsed) and a staff scientist at SOFIA (which was cancelled), Robert moved to the VLA a few years ago (and we're all praying nothing happens to that). Actually he managed a preliminary look at the data as far back as 2022, but again, stuff kept coming up, and it's taken until now to look at the thing properly.

The result (drumroll please)... they're weird objects, but they're not dark galaxies.

Betcha didn't see that coming ! Shut up, you didn't, you filthy liar.

How do we know this ? The VLA data is about four times sharper than Arecibo, and that means we can now locate the exact position of the gas much more precisely. When you're looking for especially faint optical counterparts, this matters a great deal. You can find a random starry smudge literally anywhere, so that there's always some ugly bit of stellar faff (maybe real, maybe just noise) that you can't ever be certain isn't associated with the gas. The worse the resolution, the more such faff you have to contend with. At some point you hit a wall, and getting better, sharper data is the only way to make any progress.

The VLA data reveals that in two cases, we can now identify the optical counterpart unambiguously. And they're likely neither dark galaxies nor bits of debris, but something we never expected at all.

1) AGESVC1 231

Figure 2 from the paper. The boxes show where we measured the gas content. The red and blue contours are the original Arecibo data, showing the gas at two different velocities. The inset image is a close-up of the tiny optical counterpart, just visible at the centre of the green contours (VLA) in the main image.

The Arecibo data for this is a bit of mess, but the VLA is clear : it's definitely associated with a pale blue dot. Now if I were Carl Sagan, I'd wax lyrical about how this seemingly insignificant little mote is a rich system of tens of millions of stars, perhaps host to trillions of ancient kings and emperors, a brief candle striving oblivious to the oncoming dark... but I'm not, so I won't. What I will say instead is that it's a pathetic, paltry, stupid little bugger that's bloody hard to spot.

More rigorously, while we always wondered about that tiny blue dot (I mention it right back in the earliest publications), it never seemed at all convincing. The stellar component seemed exceptionally compact, and the gas and stars weren't in a great alignment with each other (normally the positions are very close indeed). It was uniquely extreme : in other cases of small blue smudges, we could see at least some structure. With this one we could see really nothing. It just looked smooth and boring, not like a galaxy at all. And that high line width is something we'd normally expect only from very much bigger objects. It just didn't fit the bill.

But the VLA data explains all this quite nicely. Actually, as shown above, we can see it in the Arecibo data too if we'd only thought to look... but what we see is that HI has a clear tail. The densest gas appears to be associated with the stars, but the majority of the gas is actually in the stream. This is why the overall best fit to the Arecibo data gave such an offset between the gas and the stars.

So why didn't we make such a map with the Arecibo data years ago ? Largely because, as I've covered before, the existing tools to do this were a right pain to use. Remember, this is one source among more than a thousand in our full (still incomplete) catalogue... and the data really didn't suggest we had any chance of finding anything. Looking at the raw data, it's not at all obvious there's any kind of stream. Making such a map felt like a pointless and tedious exercise, surely not capable of showing anything more than we could see in the data. Maps are something you normally make only when you can already see evidence of structure and want to examine it more carefully, not to find structure in the first place.

Making maps is a classic case of something that's not actually difficult but is extremely tedious. "I'll do it this afternoon !", indeed... at least with much of the standard software.

As per another paper, I have of course since learned my lesson on this. Data visualisation and simplicity of tool use should not be considered optional extras : sometimes, they really matter !

Anyway, the new analysis also explains that high line width. Rather than being a signature of rotation, it's the result of gas being stripped out of the unusually tiny galaxy. The VLA data is less sensitive than Arecibo, but this shows us that the compact, dense gas is all associated with that blue blodge. This gives us a very nice, consistent picture of an especially small galaxy being caught in the act of having its gas very rudely shoved out. But then, it should have known better than to try and barge its way into the Virgo Cluster, the jumped-up little upstart*.

* All I'm saying is that there's room in public outreach for people who hate things and don't want to write love sonnets to their magnificent galaxies the whole time. Fuck off Carl ! Suck a lemon, Sagan ! There, that ought to get me some much longed-for hate mail...

This is still an unusual object, mind you. Not only is it especially blue, but small galaxies should lose all of their gas extremely quickly. To catch one right in the moment this happens is pretty neat, but to understand why, let's move on to the other, very different object.

2) AGESVC1 274

Figure 14 from the paper, with the VLA detection as the green contours and a close-up of the optical counterpart in the inset image.

The second object is very different from 231. Of our eight clouds, six have those peculiar high line widths, and this is one of those which doesn't : it's only about 30 km/s, a real tiddler. And its optical counterpart is extremely fuzzy and diffuse. It looks for all the world like a pretty normal dwarf galaxy, just extremely faint. 

To be fair, we actually suggested this same optical counterpart in a 2016 paper based on some deep optical data. As with AGESVC1 231's pale boring blue dot, it didn't seem like a brilliant candidate – again you can find this sort of scrappy starlight all over the place – but the VLA makes it unambiguous. This stupid fuzzy blob really is the optical counterpart after all.

I mean, two candidates I suggested turned out to be right. On the other hand, of course I also said these were shitty candidates and they were more likely to be optically dark, so more fool me.

Interestingly, this same object was re-suggested as a candidate in a paper by Dey et al. last year. They find an optical redshift* of the stars that agrees with that from the HI, which makes this association rock solid. What's more unusual is that they propose this to be a so-called "blue blob", stars which aren't forming in dark matter halos at all. They have a catalogue of 30-odd of these diffuse, blue structures which they interpret as being ram pressure dwarfs : objects which form in the tails of gas when it's stripped out of galaxies. 

* More strictly, they get the redshift from the ionised gas which emits at optical wavelengths. They don't directly measure the redshift of the stars themselves.

This is an exciting idea, essentially introducing a whole new type of object to investigate – as different from dark galaxies as dark galaxies are from tidal debris. We're talking a fundamental rethink here, not a tweak to our pre-existing ideas. And that's really nifty.

Very little indeed is known about these blue blobs, but so far they're unique to Virgo. They appear to be chemically enriched, which strongly suggests they formed within larger galaxies rather than as galaxies in their own right. Not everyone agrees, although personally I think it's extremely credible and some of the most interesting work that's been done in Virgo in years. 

Another key point is that these objects appear to lack dark matter, which seems to be the case for this one. Given that Dey's measurements show that this particular object is chemically enriched, was independently identified as a BB candidate by its images alone, and appears to lack dark matter, this all paints a nice coherent picture. Such objects could also reach large distances from their parent galaxies without the giant streams expected in the tidal debris scenario.

Wait, wait wait.... on, lacks dark matter ? I thought we were talking about dark matter dominated galaxies !

Indeed so. But of our eight clouds, only six had high line widths. Two of them, including this one, actually had widths which are if anything narrower than expected, so much so that it points to a possible deficit of dark matter rather than an excess.

There's just one problem in this case, though it isn't fatal. Unlike with AGESVC1 231, all the gas here appears to be compact, with no evidence of any stripped component. So if its stars really did form within a stripped tail, all of that appears to have dissipated. This makes it very hard to tell if we're really seeing such an exotic object or just a particularly faint but normal dwarf galaxy. 

Fortunately, there's one more object which has an optical counterpart.

3) AGESVC1 266

Figure 12 from the paper. The detection is quite clear in the VLA spectrum but only marginal visible in the map, hence the wobbly contours (dashed green are negative). White labels show possible optical counterparts : BSG is not Battlestar Galactica but the Brightest SDSS Galaxy; T2016 is something I identified previously; D2025 is that found by Dey.

Oi ! You said two objects with optical counterparts, what're you playing at ?

Not quite. I said we could identify two optical counterparts thanks to the VLA data. This one was identified completely independently (by Dey again), and to be honest, the VLA doesn't help here. Dey has that advantage of getting optical redshift data which matches the HI... without this, I would never in a million years believe their optical counterpart. 

I mean, look at it. It's pathetic. As galaxies go, it's utterly shite. It's a total miserable failure.

And that, of course, is what makes it so interesting. In both the previous cases, the mass of gas is a few times more than the stars, maybe approaching a factor of ten. That's pretty extreme... but in this case the ratio is more than a thousand. That's into crazy territory*.

* There are caveats to figures like this. Estimating the stellar mass becomes extremely difficult for things this faint, but nevertheless, it's clearly exceptionally faint – the exact numerical value isn't all that important.

A mass to light ratio of over a thousand, you say ? Might as well stick a handkerchief on your head and two pencils up your nose.

This one seems very much more convincing as a blue blob / ram pressure dwarf. Here the HI and the optical appear offset and the gas is diffuse, not compact. We may be witnessing the birth of a long-lived blue blob (a whole new class of stellar structure, as Jones and Dey and others have been investigating), or possibly just a brief flicker of star formation before the whole thing dissolves into undetectability. So birth or death ? At the moment, we just don't know.

---

What does it all mean ?

It's a bit of a mess. We have eight clouds in total and we observed six. Maybe one day we'll try for the other two, but right now we know nothing more about them. So of those six :

• One with a high line width turns out to have a tiny, compact optical counterpart and a great big stream. This looks convincingly like a stripping galaxy.
• Two have extremely faint, fuzzy optical counterparts. One of these looks like a good candidate for being a ram pressure dwarf, while the other is plausible but uncertain.
• One more was just about detected, but still has no clear optical counterpart. It doesn't look like it's rotating, but the detection is weak so we can't be confident about this.

The other two were not detected. That's expected if they lack any compact gas, which would mean they're similar to the detected-but-dark case of the last bullet point. But we should also bear in mind just how faint some of these optical counterparts can be : just because we haven't found one doesn't mean they don't exist. We would never have spotted the counterpart of AGESVC1 266 ourselves – only Dey's optical redshift allowed that. So these undetected objects could still be optically dark, but we just don't know.

Even so, we now have enough data that we can finally start to say what's going on.

Wait, wait...

I almost forgot about the legitimate clickbait version !

Twenty years ago they found dark clouds in the depths of space. We finally know what's going on.

LISTEN UP JOURNALISTS AND LISTEN GOOD. You get to say, "we finally know" if it takes nearly two decades and the answer is pretty clear. You do not get to say it if somebody comes up with a vague idea a week after a slightly odd discovery. Got that ? Good. I'm glad we had this talk.

Ahem.

Anyway, the dark galaxies hypothesis used to seem tenable because we couldn't really explain their high line widths in any other way. But we don't see any ordered motions in any of these objects, and one shows a clear stream. That means the kinematics likely has nothing much to do with the gravitational field at all, so the dark matter interpretation – which must be said was the most astrophysically exciting one – probably deserves to be chucked out the window.

I know the ejected guy is normally supposed to come up with a sensible idea, but dark galaxies did used to seem like the best explanation. Anyway I don't care, so shut up.

What looks almost certain is that we were being deceived by the apparent similarity of the clouds. It looked as though they all had about the same mass, line width, similar levels of isolation, and lack of optical counterpart. We now know that's not entirely true : the line widths vary (in some cases our new measurements reduce this compared to our original estimates) while some have optical counterparts – which themselves have varied properties. 

That means the number which we can attribute to being some form of dark debris is not eight, but maybe five at the very most... and likely less, as two weren't even observed. Couple that with the variation in properties and the debris scenario begins to look a lot more plausible. Instead of saying "they're all dark galaxies" or "they're all debris", it now looks more like some are extreme galaxies, some are new kinds of stellar structure, and some are debris. No individual explanation has too much of a burden to bear any more.

These objects are still weird though. For one thing, how come compact objects like AGESVC1 231 seem pretty good at retaining their gas even when larger ones like VCC 1964 have it all removed wholesale ? Why does it get fully displaced in one case but elongated in others ? More fundamentally, what governs when the HI gas is destroyed as it's removed from giant galaxies but can apparently survive and prosper in the harsh environment outside its friendly parent galaxy ? That's a bit weird.

And perhaps of more immediate interest are those blue blobs. In a spectacular pivot that would shame a politician, I might well find myself switching more to objects which lack dark matter rather than having too much. The objects in this paper are not, it turns out, the only such blue blobs in our data which show dynamics like this. Given that VCC 1964 shows a similar lack of dark matter, it's tempting to wonder if there's a connection between BBs and Ultra Diffuse Galaxies. Right now, all I can say is that some of our other BBs shown even more extreme gas ratios and dark matter deficits than the objects we examined here... the times they are a-changing, but the future, it seems, remains as dark as ever.

Got Gassy Galaxies ? Get Gasisgone !

What happens if your galaxy is feeling a little... bloated ? No problem ! Simply pop it into a galaxy cluster and let it dissolve slowly for a few billion years.

Say what you like about AI, it has its uses.

Okay, time for the public-outreach version of my latest paper. I'm gonna do my very best to keep this one to a readable length.

Introduction : How To Do Body Shaming For Galaxies

Back in 2015, astronomers got a nasty shock. Most galaxies, we thought, all had roughly the same surface brightness level, having about the same number of stars per unit area. Oh, sure, they varied a bit, of course, and there were a few oddball hipster galaxies that just insisted on doing things differently. But most of them were basically the same.

Then along came shiny new telescopes which were much better at finding fainter galaxies. Lo and behold, it turned out that there were plenty of such objects to be found. Now if they'd been really small tiddlers, I don't think anyone would be surprised. Dwarf galaxies do all kinds of crazy stuff anyways : we already know we don't fully understand how they work.

Like this guy, caught in the middle of two giants and causing a right mess. In some cases, interacting giants can rip off so much material from each other that whole new dwarf galaxies form. 

But the new galaxies weren't dwarfs : they were, by some measures, as large as giants. The stock phrase – and it's a decent one – is that they're the same size as our own Milky Way but a hundred to a thousand times fainter. And this is really weird, because everyone was quite happy we at least understood the basics of the biggest galaxies. The implication that maybe we didn't made a lot of people quite worried. Well, anyone would be if they suddenly realised they were surrounded by hundreds of invisible giants.

The immediate question was : ah well, yes, they're very big, but are they heavy ? Our models are mainly based on mass, not size. So maybe they're just like Zeppelins : enormous, but they might not weigh very much. And just like Zeppelins, they'd be spectacular but largely useless (or at least irrelevant for our models), and we could write off the whole thing as a Hindenburg-like tragedy except without all the horribly fiery death.

Too soon ?

Then things took a very confusing left turn. More than that, the world of extragalactic astronomy went down blind alleys, into ditches, and got thoroughly lost. We still don't know where exactly we are or where we're going, much less how to get there, but as best as I can tell, the current state of affairs is something like this : 

Most UDGs probably are those over-inflated dwarfs. They don't have much mass, but they spread it around a lot, taking up two seats on a plane not because of poor lifestyle choices or unfortunate physiology, but because they can't stop flailing their arms and legs everywhere. All their stars are very spread out, but their total mass – mainly dark matter, which we measure by seeing how fast their gas and stars are moving – is actually quite modest.

Manspreading is also surely even worse when you're a spindly fellow, and it certainly doesn't help matters if you're a contemptible fuckwit either.

That sounds not too bad, and indeed it isn't. Just like real life, body shaming the UDGs has come back to bite us, because they're (mostly) nowhere near as heavy as we might have guessed. Why, then, are we still confused ? That's because there are two major headaches nobody has yet solved. 

First, at least some UDGs are still plausibly very massive galaxies after all, which we don't know how to explain. Second... some of them seem to have far too little mass. Absolutely nobody expected this, and what's worse is that we see this even in isolation. Some of the first, most widely-reported examples of under-massive UDGs were found in groups, and these we think we can explain quite well as being the result of interactions with other galaxies (though even this isn't 100% proven). But in isolation this simply doesn't work at all, and nobody predicted galaxies lacking dark matter would ever be much of a thing. That's why we're still bloody confused.

So, too massive ? Nothing special ? Not massive enough ? The reality is likely a mixture of all three, and we're still trying to find our feet. 

If you want a more detailed introduction, I did two much longer write-ups on this. This post looked at the initial discovery while this one looked more at the first results of measuring the masses of UDGs. We still don't have much data on that. And there'll be a bunch more links throughout for those interested in more up-to-date results.

One other major issue is that we know about lots of UDGs in clusters but relatively few in the field (a catch-all term that basically means "not clusters"). So it's possible that the situation isn't so bad. Maybe most of the (more numerous) cluster-UDGs are indeed just normal dwarfs after all, but inflated by interactions with the other galaxies so they look bigger. The field objects without much dark matter could just be crazy weirdos, exotica that are extremely interesting but not actually that important in the grand scheme of things. After all, a few lunatics don't tell you much about psychology more generally, thankfully.

Essentially, we know of a few crazy galaxies outside clusters, but just a few. We have no idea of the far more numerous objects within clusters, but if they're as weird as the ones outside... well then things get freaky.

What we need, then, is to find a UDG just entering a cluster which still has its gas so we can measure its dynamics. That would help tell us if cluster UDGs are basically typical or typically weird.

We Need More Data

Which is where the current paper comes in. To be fair, there have been a few cases of estimating the masses of cluster UDGs before*, but very few indeed with data from their gas – quite possibly at the level of low single-figure numbers**. The rest have had to be done almost entirely by less direct methods, making clever inferences from globular cluster numbers and suchlike. It's very clever, but also unsatisfying, a bit like guessing someone's bank balance by the size of their house. It's nobody's preferred option. 

* One of which seems to be overly-massive for its size, but far short of being a true giant. UDGs really do seem to probe the full parameter space of weirdness.
** The paper has more on this in the Discussion section, but basically I only know of two, and even these weren't set out explicitly as UDGs.

This lack of data isn't surprising. Measuring how fast the stars are moving is extremely difficult when there's hardly any stars to measure, and as galaxies enter clusters they can lose their gas very quickly. Even if UDGs have relatively normal masses, it's not that strange that so few have been detected with gas while in clusters. 

So at last, in this paper we present a candidate for a UDG entering a cluster and still retaining most of its gas. And it too seems to have a depleted dark matter content.

Here's the data !

Credit for this discovery goes to my PhD student, who found it at the southern edge of our Widefield Arecibo Virgo Environment Survey (WAVES) data set. This is just north of the Arecibo Galaxy Environment Survey (AGES) data that I studied for my PhD, and there's a little bit of overlap – in fact it's found in both. Now characterising a galaxy as a UDGs is quite difficult since it needs such precise measurements, and while we're working on doing this for ourselves, we began by looking at a pre-existing catalogue of UDGs called SMUDGES. This one, VCC 1964*, was the only one found in our HI (atomic hydrogen) data sets.

* The name is from a catalogue from 1985. Some UDGs were found well before the big kick-off in 2015, which is when people began to recognise them as a distinct class of object.

Well then, here it is :

The contours show the gas, coloured according to how fast it's moving. The galaxy is relatively small, but you can see it more clearly in the inset image on the left. The inset image on the right shows the same galaxy as seen in ultra-violet, which usually corresponds to emission from young stars. Finally, the white arrows show the directions to two of the most massive galaxies in the cluster, and the green circle just shows the resolution of the atomic hydrogen (HI) data.

What we see here is immediately quite interesting. Normally the gas is centred almost exactly on the position of the stars (white X), but here there's a clear offset. Moreover, this same offset is seen in both AGES and WAVES data sets, so it's definitely real. The orientation of the gas and the stars is a bit weird here : we'd expect the gas to be further away from the cluster centre than the stars, but it's actually doing the opposite. That strongly suggests the galaxy has a weird orbit.

Much more fun, though, is the mass of the galaxy. The gas tells us how fast stuff in this galaxy is moving around, and the answer is... not fast enough. For this we use my favourite way to plot galaxies : the Tully-Fisher Relation.

Black circles (filled and open) show normal galaxies. The orange points show VCC 1964 as measured in a couple of different ways, while the grey points show the famous ALFALFA detections. The dashed and dotted lines show the observed scatter, with the uppermost dotted line showing five times the standard deviation – the usual criteria for judging something to be significant.

You might remember my extremely detailed run-through of how this is plotted, but no need to go through all that again. Basically it just shows the total mass of gas and stars (vertical axis) as a function of how fast the galaxy is rotating (horizontal axis). This object, much like some other UDGs with gas measurements, is rotating much more slowly than normal galaxies – suggesting it's got significantly less dark matter.

You might also notice that the mass of this galaxy appears to be much less than the other UDGs plotted. In fact it turns out to have the lowest mass of gas detected in a UDG to date, with the next highest being something like a factor five times more massive (and most much more than this). But can we then trust these measurements ? With so little gas, how can we be sure we're really measuring its rotation speed accurately ? Especially since the gas appears to have been pushed out. As someone said during the analysis phase, you can claim the gas has been removed, or you can claim an offset from the Tully-Fisher, but can you really claim both as important at the same time... ?

That is definitely the most uncertain bit of our analysis. We've found previously that galaxies do deviate from the Tully-Fisher simply when they've lost enough gas, with no implications for their dark matter at all. That can happen because the outermost gas is both the most easily removed and rotating the fastest. Strip this away and the measured rotation speed will shrink, but this doesn't tell you anything about the dark matter content at all.

A simple simulation of ram pressure stripping. Dark matter (not shown) is completely unaffected. But since we infer dark matter by the rotation of the gas, if the fastest-moving gas is removed, it can look as though the galaxy has less dark matter than usual.

It's certainly possible this is the case here, but there are mitigating factors. We found that large deviations only happened when galaxies had lost a lot of gas, but in this case the gas loss only seems to be very modest : actually what we're seeing is more gas displacement than loss. All of the gas appears to have been shifted in bulk away from its happy place; effectively all of the gas should count as the green "disturbed" component in the above diagram. 

This makes the interpretation tricky. We can say quite confidently that the rotation hasn't been reduced just due to sheer loss of the fastest-moving gas. The problem is that we have almost no clue what happens when you get a wholesale bulk movement of the gas like this – it might disturb the gas in other ways we haven't accounted for. There just aren't nearly enough other examples like this for us to make a comparison.

And we also tried another plot of the TFR, this time not using total mass but just optical brightness. This is subject to less corrections that have to be applied to the data, and we found an even stronger deviation : at least six times the scatter in the normal galaxies, and possibly even more than this. So two different plots by two different methods gave us the same result, which is pretty neat. And in a few other cases, it's been shown that the velocity dispersion actually increases when gas is stripped, not decreases. 

What's Going On ?

VCC 1964 has a great deal of collective weirdness :

• Its gas is displaced from its stars, but little is actually missing : it has the lowest mass of gas in any UDG detected to date, but that's consistent with it being a right little tiddler (not because a lot of gas has gone missing).
• The offset between the gas and stars suggest a weird orbit, with most of the motion across the sky rather than along our line of sight.
• The apparent rotation speed is quite a lot lower than expected. This can't be accounted for by gas loss or measurement errors, but we don't know if gas displacement by itself could also cause this effect.
• And what I haven't mentioned is that this galaxy is both smooth and blue. This combination is itself very unusual : more often, blue galaxies have lots of structure and star formation. It might be related to the galaxy just having lost all of its gas, with enough time for the structures to smooth out but not enough for the colour to change. Maybe. It's also strange that removing the gas, which is probably about as massive as the stars, doesn't appear to have altered the stellar structure at all.

What if it's all just a horrible coincidence ? Could the gas actually have nothing to do with the stars at all, with both at different distances ?

This has actually been proposed to be the case for another possible example of a UDG losing gas in the Virgo cluster. Personally I'm a bit skeptical about this, but it's possible in that earlier example. But to find two examples of UDGs in Virgo where the gas is just coincidentally aligned with some unrelated stars... nah, I simply don't believe it. And we looked very carefully to see if there are any other possible sources of the gas nearby and there just aren't. I tend to rule this idea out.

What we know with some confidence is that this is a UDG caught in the act of losing gas. That's already extremely unusual, and we must be seeing it at a very precise moment indeed for it to be both blue and smooth. What's much more uncertain is whether shoving the gas out of the galaxy would cause the illusion of a lower rotation speed : in other words, does it really not have much dark matter, or is this just because it just got kicked in the backside ? Having little dark matter would be consistent with other UDGs, and – maybe – also help explain how come the gas can been removed in bulk, wholesale, rather than dragged into a long tail as is more normal.

And even more speculatively... what does this mean for the screaming hordes of UDGs in clusters more generally ? Are they just giant collections of stars rather than more typical, dark matter-dominated galaxies ? We just don't know, and intuitively, it's hard to see how they could survive very long in a cluster... though it seems they do tend to avoid the densest parts of clusters, so maybe they simply don't. Maybe they just get torn to shreds. 

All we really know is that everything is very confusing.

The final question is whether objects like this are connected to the so-called "blue blobs". In my opinion these are some of the most interesting objects found lately, though I haven't talked about them much. They're... umm, well, they're fuzzy blue things which don't quite resemble ordinary galaxies, and there's been some very interesting suggestions that they might form in the gas stripped out of larger galaxies by ram pressure. They too would lack dark matter. So is there a connection ? Is VCC 1964 actually a very large blue blob ? Or will it even spawn a blue blob from its own stripped gas ? How do all of these relate to the dark galaxies I more usually go on about ?

Well, good news ! Our paper on using the VLA to get higher resolution on those dark clouds is damned close to acceptance. And we have more observations accepted with the VLA for VCC 1964 itself, which should give us a much clearer picture of what its gas looks like than the big blob we see in the Arecibo data. More research is needed... but this time, more research is actually happening.