Ultra Diffuse Galaxies : Revenge Of The Ghosts

Why is everyone wearing badly-ripped trousers lately ? Like, literally almost everyone, men and women alike ? If it's supposed to look sexy, it doesn't. It just looks daft. Just like when everyone in the entire world was wearing ponchos a few years ago for no good reason other than that everyone else was wearing ponchos.

Scientists aren't really know for being particularly fashion-conscious and those that do tend to go to bizarre and questionable though strangely wonderful extremes... at least when it comes to clothing. As far as research goes, however, it's a bit of another matter. There's generally almost always someone working on just about anything, but every so often something happens that transforms the dull, plodding research in some particular field into a crazy festival that's overtaken by screaming hordes of sexy young people (and often screaming hordes of drunken academics as well).

Currently the winner of the sexiest, most fashionable topic in extragalactic astronomy is undoubtedly the analysis of so-called "ultra-diffuse galaxies". You might remember I wrote about these almost two years ago when they first started hitting the headlines. For a topic this large, it's usually not worth paying too much attention to individual press releases. But two years is enough time that many, many more studies have been done, so it seems like it might be worth an update. First, some background history.


Bright Galaxies Are For Losers

For a long time it was thought that all galaxies had the same "surface brightness" - that is, they seemed to emit the same amount of light per unit area. This was a bit surprising, but then along came the discovery of Malin 1.

Malin 1 isn't a particularly faint galaxy, but it's much larger than other galaxies of the same brightness. A trickle of other giant "low surface brightness" galaxies followed over the years, but none quite so dramatic. A few people thought it might just be a selection effect - we were only seeing galaxies that were so bright that they were easy to detect, and that only much more sensitive observations would reveal greater numbers of these LSB galaxies. Other people thought that these LSB objects were just exotica - sure, Malin 1 might be super interesting, but not all that important in the grand scheme of things. Kindof like the giant squid : sure it's cool that there's an enormous version of the regular squid, but it lives in the deep ocean and literally no-one thinks about it on a daily basis except for hungry sperm whales and, presumably, other giant squid.

And daring sea captains, of course.

While the giant ghostly galaxies were sometimes viewed as nothing more than a curiosity, it was a different story for LSB dwarf galaxies. Now, to you or I dwarf might mean something that's physically small, and giant something that's physically large. Not so in astronomy, where the terms don't have strict meanings but are generally used to refer to brightness, not size. If astronomers ran hardware shops, they'd call 20W bulbs "dwarfs". That's the kind of linguistic logic we're dealing with here : no wonder we have such a stupid definition of "planet".

Anyway, like flies on a windscreen, LSB dwarf galaxies were found in large numbers relatively quickly. By and large, dwarf galaxies do tend to be smaller than giant ones, and they're also much more common. This means you don't need a large field of view to discover them. What you need are two things : 1) Long exposure times (nothing difficult about that) and 2) a method to estimate their distances. You need the distance to work out their true size, mass, and brightness, otherwise you have the classic problem of not knowing whether your cows galaxies are small or far away.

Directly measuring distances is hard, but there's a trick that makes things much easier - look for the faint galaxies in known clusters where the distance has already been measured using bright galaxies. If you see more of them in the cluster and less in a control field, you can be pretty sure that most of your discoveries will be at a similar distance. Of course, not every object will be a cluster member, and even those that are won't be at exactly the same distance - but it's a reliable enough method that your conclusions will be statistically valid.

LSB dwarves were so easy to find that they moved from the realm of "neat" to "dull" pretty quickly... at least in terms of discoveries. They became routine. They didn't stop being interesting, but... where were all the giant LSB galaxies ? Eh ?


Moaaar Galaxies ! More I Say ! Hang Those Who Talk Of Less !

That answer only came much more recently, with hundreds upon hundreds of giant LSB galaxies being discovered using more sensitive telescopes with the wide fields of view needed to detect the giant objects. Far from being exotica, they seem to be extremely common. Some clusters contain hundreds of the ghostly blighters, although in other cases their numbers are far less.

From one of the first discovery papers.

From another early paper of the heady days of 2015.

Finally some more interesting-looking objects from a more recent paper.

As you can see, most of these galaxies aren't visually very appealing (though there are some interesting exceptions). Mostly they're smooth, boring looking objects. They don't seem to have much in the way of gas or star formation - they're just red, dead pustules of stars floating through the universe like long-dead corpses, or something.

Almost all of these galaxies have been discovered using the same trick as for the LSB dwarf galaxies : looking for faint smudges that seem to be more common around clusters of known distance. We've also now got a few more accurate distance estimates, so we're pretty confident that the majority of these discoveries really are giant galaxies far away. Of course, no doubt a few of them are small objects nearby, but the majority are definitely a long way off.

Make no mistake : this is really cool. Hundreds and hundreds of giant, faint galaxies we never knew existed ! But... what is the nature of these objects ? How do they form ? Are they just like regular galaxies except that they never formed as many stars because they got bored, or did something happen to them to stunt their emotional growth ?


The Nature Of The Beast

Galaxies generally have several components : stars, gas, dust, and dark matter*. Technically all of these are optional, but usually ya gotta have dark matter. Dark matter dominates the mass of most galaxies. Gas and dust are completely optional : we know of loads of cases where galaxies just have stars and dark matter. They're really, really boring, but most of the Universe is so damn complicated we should be frickin' grateful for a little boredom once in a while.

* To keep this short I'm just gonna assume dark matter is real; for more details see these posts.

So frickin' boring.

Dust rarely (if ever) makes up much of the mass of a galaxy, but the gas can be significant - sometimes there can be far more gas than stars. This raises the question of how extreme these "dim galaxies" can get... instead of low surface brightness, could we have galaxies of no surface brightness with just dark matter and maybe some gas ?

Perhaps. That's a hugely controversial topic, which the enthusiastic or drunken reader can examine here or here. But it also makes us re-examine that definition of "dwarf". Leaving aside the gas and dust, we can measure galaxies in terms of their size, mass, and brightness. Traditionally we've been able to use brightness as a pretty decent proxy for the other two, since there seems to be a nice correlation. So calling a dim galaxy a dwarf wasn't totally mad. But these new "ultra diffuse galaxies" are very different to almost all other galaxies we've seen before : as large as the Milky Way but a hundred or a thousand times fainter.

What we'd really like to know is their total mass, i.e. how much dark matter they have. Then the question becomes : are these huge dwarves (big but not very massive) or ghostly giants (big and massive but very dim), a.k.a. "failed galaxies" ?

If you stuck a bicycle pump up Gimli's backside and applied sufficient pressure, you'd get a huge dwarf. His mass wouldn't change, but his size would.

For cosmological models, it's their total mass which is the most interesting thing. Such models traditionally only use dark matter because it makes the calculations much easier, which means they have limited predictive power when it comes to things like brightness and size. But mass they ought to be able to get right. They've been plagued with problems for the lowest-mass galaxies, predicting far more tiddly little dwarf galaxies than are actually observed, but it's looking increasingly likely that this was just because they didn't include the complicated physics of the gas and stars. So there could be plenty of small galaxies which just have too few stars to be detectable.

But these models haven't had a problem when it comes to massive galaxies : they get their numbers about right. If these new galaxies are low mass, then they could help solve the missing galaxy problem (or at least not make it any worse). If they're high mass, then...

Unlike in popular 80's movies, a few giant ghosts wouldn't spell disaster. That could just mean that we don't properly understand how gas forms stars, which we know we don't understand anyway, or how gas gets into the dark matter clumps (which we also don't know anyway). So they could even help us understand the missing midget galaxy problem. But if all these giant ghosts are massive ? Well then we've got a real problem.

Who Ya Gonna Call ? Some Dorky Astronomers, Probably, Because This Is An Astronomy Problem

Following this field over the last couple of years has been quite exciting. Sometimes the evidence swings one way, sometimes another. Currently, the balance of probabilities is shifting towards most of new giants being huge dwarves than giant scary ghosts... but the dwarves aren't having it all their own way.

Normally we can measure the total mass of a galaxy by seeing how fast it's rotating, or how fast the stars are buzzing around if they're not in rotation. We can do this by measuring the stars, but it's generally reckoned to be more accurate to use the gas. Gas is normally more extended than the stars so it lets us measure how fast the galaxies are rotating at greater distances from their centres. Unfortunately, most of these new ones don't seem to have much or any gas, so we have no choice but to resort to the stars. And that sucks, because these things are so bloody faint that measuring the speed of the stars is abominably hard. So two years later, we don't have that many mass estimates.

But we do have some. The first measurement was of a pathetic galaxy known as VCC 1287. There had been speculation that ultra-diffuse galaxies in clusters would need particularly high dark matter contents to survive being ripped apart by the other galaxies. These measurements confirmed that : this object has far more dark matter than other galaxies of a similar mass. But not so much that it could be classed as a giant, which is quite satisfying.

It's either an interesting galaxy or someone sneezed at the wrong moment...

To get around the problems of measuring this miserably faint smudge, the authors (a certain Michael Beasley et al.) used a neat trick - they measured its globular star clusters instead. Globular clusters are known to surround many galaxies in a sort of cloud. Because they contain large numbers of stars in a small area, they have higher surface brightnesses than the rest of the galaxy, making them much easier to measure. So the authors were able to use these to directly estimate how fast the galaxy is rotating*.

* Strictly speaking it's not rotating, it's swarming. The point is to know how fast the stars are moving. Whether they're rotating nicely or moving in crazy random orbits, the method is the same : speed => total mass.

But they were also able to do another clever trick. Using a known relation from other galaxies, they were able to estimate the total mass just by (in effect) counting the number of globular clusters. And they got a result that was in great agreement with their direct measurement. Hurrah !

The grey circles are VCC 1287; all the rest are a bunch of other crappy galaxies. Total mass in on the y-axis, stellar mass on the x-axis. See how VCC 1287 has way more total mass than most galaxies with that many stars ? We'll come back to that later.

Now, a bit of caution is needed because that globular cluster versus total mass relation wasn't previously calibrated for galaxies like this. But it's in such good agreement that it suggests that we can - tentatively - use it to explore the masses of other UDGs. And we don't even have to do the tricky work of measuring their speed (which is complicated), we can just count them (which is way easier). So subsequently they repeated this neat trick on another ultra-diffuse galaxy, and found that that one also is probably a huge dwarf instead of a giant ghost.

Thus far most of the UDGs we've see have been the Brienne of Tarth of the galaxy world : very interesting, but not terribly attractive. One of the rare things that astronomers agree on is that without star formation, galaxies quickly settle into the photogenically uninteresting "red and dead" phase. There are several reasons for this. First, stars are so small that they rarely collide or interact with each other except through gravity, so a collection of nothing but stars is quite well approximated by a swarm of angry bees. There's nothing to make the bees stick together, so they form something quite smooth and uninteresting to look at unless it's chasing you because you raided its hive... the analogy probably breaks down at that point.

Second, blue stars are short lived. So once star formation stops, the blue stars quickly die off and the galaxy becomes a boring homogenous red colour. And for star formation you need gas... which also happens to make things more interesting for another reason. Let's switch from bees to cats. A swarm of angry cats is pretty well randomised, but imagine a swarm of cats following catnip : much more interesting. Gas can collide with itself and stick together to form interesting structures, and within those denser structures stars can form. Gas, essentially, makes galaxies beautiful. Maybe someone should tell the make-up industry.

The first hints of UDgs with gas came indirectly, with the discovery that some of them were blue and had complex structure :

The authors of that study found that the redder ones had less stars and were overall rounder. And the bluer ones seemed to be more common in the outskirts of galaxy groups. So perhaps as they fall into the group and get harassed by the other members, they can lose gas (and some stars) and transform into the boring red objects that dominate in most cases. They also found that the UDGs were found in similar locations to the normal dwarves, suggesting that maybe these UDGs were over-inflated dwarf galaxies rather than giant ghosts.

What we'd actually like to test this is a direct measurement of the gas, not just inferring its presence by their colours. That would also let us directly measure their total mass.


Gas for some... miniature American flags for others ?

Cometh the hour, cometh the archival data. The huge ALFALFA survey is covering the whole sky visible to Arecibo, looking for hydrogen gas rather than optical galaxies. With over 24,000 galaxies catalogued to date, it's an excellent resource to mine for UDGs that may have been observed but not noticed. Which is exactly what the authors (led by PhD student Luke Leisman) of this study did, combining their results with the even larger optical Sloan Digital Sky Survey.  They found about a hundred UDGs hiding in the data, all with gas.

Indeed, "woohoo". Mostly. But there are some important limits. Except for three cases, the resolution of the gas observations isn't very good. This means they can't estimate the total mass as well as they would like. Normally with observations like this you can make pretty reliable corrections, but these galaxies are so faint that's not really possible.

Sometimes limits aren't necessarily bad. A more interesting restriction is that all of these sources are isolated, much more so than the previous results which were all in groups or clusters. That's a limit the authors imposed to avoid confusing their discoveries with other sources. They were also able to completely avoid the need to look for galaxies in clusters, because with hydrogen surveys one gets the distance measurement directly from the data.

What they found was that these galaxies are blue and irregular... and low mass. Although the mass is very uncertain, it seems unlikely that most of them are giant "failed galaxies" - they are indeed just over-inflated dwarves. So, galaxy formation theories saved then ?


Things are indeed getting weird

Errr, well... ummm... dunno ! We need to make a brief digression here, because these galaxies also have another especially weird property : they seem to be rotating much, much more slowly than expected. This plot wasn't in the paper, and I'm hesitant to do science by blog post, but it's the best way of illustrating it :

You can see that normal galaxies follow a nice clear trend : the faster they rotate, the higher their total mass. This is not so for totally optically dark hydrogen clouds, which rotate much more quickly than normal galaxies of the same mass. But the ALFALFA UDGs do the exact opposite, apparently rotating much more slowly than regular galaxies.

This is something I've covered in more detail here, but the main point is that measuring rotation for the hydrogen clouds is difficult - we don't really know if they're rotating or not, so we may not be making a fair comparison by plotting them together with normal galaxies. But for at least some of the ALFALFA UDGs, that's not the case. Three of their objects (the ones with the high resolution observations) really do look like they're rotating. And while we know that it's possible to create the appearance of slow rotation in galaxy clusters, these galaxies are bloody miles away from any others, so that can't be the explanation in this case.

So this result is potentially super-interesting. In decades of observations, no-one's ever seen a deviation like this before... and that's been a surprise, because it's not obvious from theoretical grounds why there should be such a neat relation between rotation speed and mass. We expect galaxies of low surface brightness to deviate, but so far they haven't. Could it be that there isn't really a neat relationship after all, and it was all some kind of weird selection effect ?

I asked Lesiman about this, and he gave a very cautious response - perhaps even a little too cautious. He says that there could be a biases in the selection procedure might mean that the rotation widths are underestimated. The above plot might not be comparing widths in the same way. Maybe the galaxies aren't really discs at all. Or we might be missing the fastest-rotating objects because of sensitivity effects. I agree with most of these, but I think the effect would be weak - even combining them all, I doubt it will change the end result.

To show just how strange the result is, it's worth noting that these pathetic objects...

... all lie slap-bang on the "Tully-Fisher" relation. Even the crappy ones which are certainly LSB dwarves (though they're not UDGs, which are larger). It's worth remembering, though, that we have hardly any measurements of the UDGs, so we have no idea where the majority lie on the Tully-Fisher plot.


But at least the UDGs are dwarves, right ?

Probably. The bulk of the evidence looks to be pointing that way, and there's at least one paper showing how dwarf galaxies could become so large in principle (whether or not that actually would happen according to the standard models is another matter). But there is some dissent. Enter the Dragon...fly 44 galaxy.

Dun, dun, duuunnn !

I've already written a little bit about the galaxy Dragonfly 44, noting how although the press release claims it to be the first discovery of a "new class of galaxy", the reality is more complicated. Yet there's no doubt it's a very interesting object. Unlike the other objects it does seem to be a "failed galaxy", with as much dark matter as the Milky Way. And the authors were able to estimate the mass in two ways - from the velocity of the stars and the globular clusters, and by counting the globular clusters. The results are in good agreement.

... but, let's not be hasty. There's considerable margin of error in all of the estimates of total mass, and not just because of measurement uncertainties. The authors note :

We emphasise, however, that the halo abundance matching technique relies on total halo masses, and in our study the total halo mass is an extrapolation of the measured mass by a factor of ∼ 100. A more robust and less model-dependent conclusion is that the dark matter mass within r = 4.6 kpc is similar to the dark matter mass of the Milky Way within the same radius.

The difficulty is that you can only directly estimate the mass of dark matter from the visible matter. You don't really know if the dark matter cloud (or halo) extends much further, because if it did it wouldn't change the measured velocities.  The way the total amount of dark matter is estimated is by comparing it with numerical models... models which are known to have some pretty nasty problems.

So the direct mass measurement of this galaxy is consistent with the Milky Way as far as it can be measured. The visible light continues out to greater distances but it's too faint to use for measurements. If the usual relation holds, then Dragonfly 44 may indeed be a giant stupid galaxy that's failed at forming stars. This is not improbable, and that the globular cluster/velocity measurements have given results in good agreement with each other for VCC 1287 bodes well. And the authors also note :

Better constraints on the halo masses of UDGs may come from lensing studies of large samples. Intriguingly, a weak lensing map of the Coma cluster by Okabe et al. (2014) shows a 2σ peak at the location of Dragonfly 44. Peaks of similar significance have inferred masses of a few trillion solar masses, and unlike most other features in the map it is not associated with known bright galaxies or background structures.

Gravitational lensing can provide an additional, independent mass estimate of the entire halo. The evidence for this effect for Dragonfly 44 is weak, but intriguing.

Still, we should be cautious about that large extrapolation. And a recent paper has also found that galaxies experiencing gravitational forces from their neighbours can have large amounts of their mass removed without changing their size. How much of an effect this could have in a galaxy cluster like the one in which Dragonfly 44 resides isn't yet known.


Oh yay, more boring galaxies. I shall try to contain my excitement.

Not content to rest on the exciting laurels of Dragonfly 44, the van Dokkum-led team have analysed a bunch more galaxies in the same cluster. They found that although Dragonfly 44 is still the star of the show, 15 other galaxies could also be these ghostly giants, based on the number of globular clusters. And they also show that for those three cases where we can estimate the dark matter from velocity measurements as well as cluster counts, the galaxies lie on the usual relation. That suggests the cluster count method is reliable, even though these galaxies are so different to the usual ones.

But they also comment on the previous suggestions that most UDGs are actually huge dwarves, and here we come a bit unstuck. They note good reasons why there are differences between different studies, especially why they're finding more globular clusters than everyone else - but they don't say which method is better. One important detail is that the number of globular clusters has to be corrected for background sources that aren't associated with the galaxies, but it's not at all clear who has the better method. So I think they're jumping the gun when they claim, "The results presented here... put to rest the suggestion that most cluster UDGs are directly related to smaller galaxies of the same total luminosity." Pretty strong words*, and not really justified as yet given all the other evidence, in my opinion.

* A direct translation from academic to normal speak is difficult but it implies something like, "everyone else is wrong, shut up already."

The other worry is that it seems to be van Dokkum Against The World. It seems to be only this group that's claiming these are failed galaxies : everyone else seems quite convinced that they're inflated dwarves. It's far too early to use this as evidence against them (and I should mention that if van Dokkum is right then it supports my own research, so let's no-one cry out "bias" please because it wouldn't make any sense), but it would be nice to have an independent claim for the giant ghosts.

Late one night, two teams of astronomers are locked in a house haunted by a particularly angry dwarf galaxy.

Summary

Two years on, and it might feel as though we don't know that much more than when we started. Certainly the main issue - how massive all these new galaxies are - remains controversial, and could easily take another two years or more to settle. Considering how incredibly faint these galaxies are and the pressures of getting telescope time, it's really not so surprising that this is proving difficult.

But even if the full implications of these new discoveries have yet to be worked out, we are learning things :

• We know that giant ghostly galaxies are not the rarities we once thought they were, they're very common.
• We know they're found in all galaxy environments, from isolated objects up to rich clusters. We also think that some clusters are richer in UDGs than others.
• Although UDGs have far less stars than normal galaxies of the same size, they apparently have even more globular clusters. Whatever stops them forming stars in their main body doesn't stop them forming clusters !
• The gas content of the galaxies is highly variable. Malin 1 has the highest known gas content of any galaxy ever (it's got like, literally tonnes of the stuff), whereas most of the others seem to have no gas at all. It doesn't seem to matter how bright they are : some faint galaxies have no gas whereas others have more gas than stars.
• UDGs come in many different shapes and sizes. Generally they have about as many stars as a large dwarf galaxy, but their size can vary considerably. Most seem to be smooth spheroids. Some are irregular. A very few, like Malin 1, are giant spirals.

So we've learned a lot, but we're still in the phase of learning more about our ignorance than anything else. We had no idea these things even existed until a couple of years ago, and now they seem to be an extremely important component of the Universe. How much of a challenge they pose to our theories is yet to be determined. 

We know of different possible formation mechanism for these objects and it's likely that all of them play some role. The question is, which is dominant ? If they're giant galaxies, then why did they form so few stars compared to other galaxies ? What mechanism affected them that didn't affect normal galaxies ? Why do some have huge amounts of gas but apparently little or no star formation whereas others are completely gas-free ? Why do some clusters have more UDGs than others which contains lots of very faint dwarf galaxies ? How much fainter can galaxies get - could some have no stars at all ?

We're only just beginning to tackle these questions, and more besides. As usual, observation is running ahead of theory. Not only does the question of whether they're huge dwarves or ghostly giants continue to cause controversy, but we also don't yet know exactly how many there are. Finding these things requires no just deep observations but also robust statistical models. Most detection methods use automatic techniques to find faint smudges in clusters that aren't found in control fields... but these automatic catalogues typically start with numbers in the millions that are selectively cut down to a few thousand candidates. This doesn't mean there are millions of undetected galaxies, but it does mean there are almost certainly a lot more undiscovered objects out there. So stay tuned, watch this space - the best is surely yet to come.

So spooookyyyy....