You should have gone before we came out

Where do galaxies lose their gas ? Galaxies in clusters have been shown to have much less gas than in the general field. In Virgo, for instance, on average they have ~50% or less than comparable field galaxies of the same morphology and brightness. 

Now we know there are lots of processes at work inside clusters themselves than can do this, especially ram pressure stripping. That's almost certainly responsible for the majority of the gas lost. But there are many hints that galaxies elsewhere can lose gas too, not usually as much as in clusters, but enough to be significant. This is important because most galaxies don't live in clusters, so if we really want to understand galaxy evolution, we should probably stop spending so much time on the sexiest 1%* of the population and look at all the others from time to time.

* Well, it might be a few percent, but not more than this.

This is another paper I wouldn't normally read because reprocessing existing data tends not to accomplish much. But in this case I think they're on to something... ironically by looking at clusters. Though I have to say, their sample definition seems convoluted in the extreme and they gave such a detailed breakdown of how they divided everything I wanted to slap them and shout JUST TELL ME THE DAMN NUMBERS ! Which they eventually do, and it's a few thousand per object type.

Key to this is that they can distinguish between different sorts of galaxies in clusters. I was racking my brains because this seemed curiously familiar. In fact this idea was mentioned a couple of years ago in a conference, but disappointingly the presenter isn't on the author list or cited. It may, of course, be a completely independent discovery.

Anyway, most galaxies in the field are in small groups, often very small : two or three members, though sometimes more. So this means that clusters tend to assemble by absorbing whole groups of galaxies rather than individual ones. In some cases it's possible to identify subgroups within a cluster, which are therefore likely recent arrivals. Individual galaxies within clusters, not part of any subgroup, could be individual field galaxies falling in for the first time, but they're more likely to be older arrivals whose original constituent group has been broken apart by the chaos of the cluster. 

By analogy, nightclubs. It takes a while for groups of people to get broken up by the general throng, so if you see a group of people all together, chances are they just arrived. And equally, not many people go to nightclubs by themselves, so solitary people looking lost and confused have probably become unfortunately separated from the group that originally dragged them in there.

But... galaxies. What the authors do here is very simple. Using samples that identify these different galaxy types, they plot (see their figure 1) the fraction of star-forming galaxies as a function of cluster-centric distance. They find that for solitary galaxies in clusters, the star-forming fraction steadily increases as you go further from the cluster... right up to the level of the general field. There's no distinct break in star formation activity for individual galaxies as they enter the cluster, it's just a smooth curve.

In contrast, galaxies which are still in sub-groups within clusters also increase their fraction of star-forming members, but they reach a plateau. At high enough distances, the star-forming fraction never increases for these galaxies, whereas for the solitary ones it just keeps rising.

In other words, the gas-rich, late-type (spirals and irregulars) individual galaxies just continue losing gas as they get ever-closer to the cluster centre. But exactly the same type of galaxies which are in groups never had as much gas to begin with. They've already lost some of their gas. Not as much as they eventually lose in the cluster, but still a detectable difference compared to individual galaxies. Ergo, pre-processing definitely happens.

I think this is a very nice confirmation of something already strongly suspected. The main accomplishment here is using a substantial sample to increase the cluster-centric distance well beyond what was previously attempted, which explains why it wasn't seen before. You really need to go to very large distances indeed to see this, but when you do, it's astonishingly clear. Honestly it's rare and gratifying to see a result in astronomy which is so clear-cut as this. Hooray !

One small caveat : you might be wondering, well, shouldn't the individual galaxies still show a plateau if they too tended to be in groups to begin with ? Probably not. Deep within the cluster all galaxies will be dominated by cluster-specific processes. But at the distances where there's a difference between individual and group galaxies, it's probable that the solitary objects were never in groups : they're just too far from the general melee of the cluster to have been much affected by it. So there is a change in the nature of individual galaxies as you descend into the cluster, from being dominated by the habitually-solitary to those who were indeed once in a group. But both of these end up being equally affected by the cluster's ram pressure, hence the smooth, continuous change in star-formation properties.

Do legumes give galaxies gas ?

This paper is about "green pea" galaxies, which are so-called because they're small, highly concentrated and even look green because of their strong spectral line features. Green is a very rare colour in extragalactic astronomy, and indeed for stars in general. The blackbody curve over which they emit means that they emit so much light across the whole spectrum that green is always washed out by the blue and the red. Only when you get something that's not a blackbody, something emitting over a very narrow wavelength range, do you get anything green. And that's rare.

"Green peas" tend to be more distant, but there are also closer "blueberries", which you'll have guessed are much the same apart from the colour. Whether they have similar spectral line emission to green peas is not clear.

Anyway, the authors here try and figure out how the HI (atomic hydrogen) gas content of the green pea galaxies varies with their stellar content. A perfectly sensible goal given how strange these objects are; perhaps this could shed light on star formation in extreme objects. But I have to say I don't like it very much.

Like some other papers I've been reading lately, this one contains no new observations but uses entirely archival data. But unfortunately this one is more typical of its class, not actually finding anything new or at most a marginal, incremental difference. They have a sample of 19 HI detections (and 21 upper limits, which were observed but not detected), though they have to exclude a couple because of issues with the optical and/or UV data. Based on the details they go into, they seem to have re-processed the HI data, though it isn't at all clear why – was there some issue with the earlier analyses ? If so, they don't mention it.

Figure 2 is by far the most interesting. This plots the now-classic "main sequence" of galaxies showing how their star formation rate scales with stellar mass, in a nice, tight correlation. Green pea galaxies also seem to correlate but with a totally different, much steeper relation : they have far higher star formation rates than their stellar masses predict. This holds true for both the HI detections and non-detections. Unfortunately the upper limits of the non-detections are all over the place, so it's not possible to say if there's any real difference between those with and those without gas. And the main trend is not a new result.

What they instead concentrate on is finding other relations to the optical data. They say their sample is offset from a previous relation between the MHI/M* relation as a function of NUV (near UV) - r magnitude (basically a colour measurement*), but... well it might be, but to me that earlier relation, for normal galaxies, itself looks like a dodgy fit. So I don't think too much can be said here.

* One minor but quite interesting point they make is that using this UV-optical is better than using two optical wavebands for colour, since the green pea star light is much more UV-dominated than in normal galaxies.

One thing they do show which looks convincing, but I'm not sure if it's a new relation, is that green peas have excessively high gas fractions as a function of just about any parameter. New or not that's nice, but even here their plot could easily have been so much clearer. Then they try and plot gas fraction offset (measured – expected) as a function of various parameters, say there's a trend, but as far as I can tell there just isn't. This is one of those cases where if someone shows you a weak trend and you say, "I've seen worse claims"... this is one of those worse claims.

Then they try and find what sort of scaling relation does the best job of predicting the HI mass of green peas. As far as I can tell, they seem to have taken the standard practise of throwing everything against a wall and seeing what sticks a little too far. They come up with hideous relations involving different colours, stellar masses, star formation rates and surface brightness levels that to me has no obvious physical significance at all. Frustratingly, they don't discuss this. 

And that I do find strange and annoying. It's reminiscent of p-hacking, where if you search for correlations using complicated enough relations and large enough data sets, you're bound to find something. Was there any prior reason to suspect this torturous relation had some physical significance ? If so then it's interesting ! If not, if, as I suspect, it's just an empirical fit, then it's just a statistical artifact. I'm not saying that's the case, only that they needed to be a lot clearer about what this is supposed to mean, physically, before I take any further interest in it.

It's gotta come from somewhere

One thing that's never seemed terribly puzzling to me is where galaxies get their gas. Estimates of the star formation histories show that with their current gas content, galaxies should typically exhaust their gas within a gigayear or so. A lot of people find this suspiciously fast, that to maintain their currently constant levels of star formation is impossible unless the galaxies are being re-fuelled from somewhere.

Personally I've always found that one gigayear is not fast enough for a need to invoke refuelling (or accretion as it's usually known). It just means that galaxies had more gas in the past and will run out comparatively soon, but in a time equivalent to about one-tenth the age of the Universe isn't enough to set alarm bells ringing for me. Sure, if it was the next few or tens or even hundreds of millions years, then I might be concerned. Then I might say, "hang on, isn't it a weird coincidence that we've arrived on the scene just as galaxies are about to stop forming stars forever ? Isn't it more likely they're being replenished from somewhere ?"

On the other hand, the apparent constant rate of star formation does seem more legitimately odd. If galaxies are truly running out, naively you'd expect to see that reflected in their star formation histories. So people have postulated that galaxies are accreting gas from the field somehow, either from "hot mode" where gas cools very slowly and omnidirectionally, or in "cold mode" where it condenses into cooler, more distinct streams which funnel themselves into the galaxies. Claims to have detected the latter are always controversial because it's very hard to say that a stream of gas isn't just gas that the galaxy is losing by a host of much more well-understood processes.

Today's paper attempts to address this mild puzzlement. They use a sample of galaxies which is as homogenous as they can get and apply some reasonable scaling relation where necessary to calculate the change in gas. For instance, they assume that the bulk of the normal, "main sequence" galaxies today were already on the main sequence 4 Gyr ago. That is, they formed stars at a predictable rate given their total mass of visible matter. This is not at all an unreasonable assumption : 4 Gyr is enough to expect some evolutionary changes but nothing dramatic, and while of course plenty of individual galaxies might have experienced the odd burst or sudden cessation of star formation, there's no reason to think these would be statistically significant. 

And kudos to the authors for clearly acknowledging their assumptions and how complex real star formation activity can be. In public talks I sometimes go on a breathless monologue describing the various process at work and how they relate to each other; they do much the same here, except that they clearly spell out how this is likely to affect star formation – rather than my own take-home message which is only that it's bloody complicated. 

The really big assumption, the most difficult point to make reasonable inferences about, is how much the HI gas has changed. A handful of stacked observations have now managed to detect this atomic gas out to these vast distances, but these combine data from hundreds of galaxies. We currently have no real idea how it's changed in individual galaxies on these timescales; in contrast, the molecular gas can be observed directly and is much better understood. 

Add to that that the relation between atomic gas and star formation appears to be subtle. It's popularly described as the galaxy's fuel tank, the reservoir from which star formation ultimately occurs. Which is quite appropriate, since understanding how much gas is in the tank and how fast a car is going is not easy ! The actual gas in the engine itself, the stuff that's exploding in the pistons, is thought to be molecular, but there are strong hints that HI is directly involved as well at least in some cases. Most HI-rich galaxies are blue and star-forming, but some have loads of gas but hardly any stars at all or only old red ones, so it's not at all a straightforward connection.

Using their various scaling relations as best they can, their conclusion is at least an interesting possibility. They say that what seems to be happening is that galaxies are both losing and gaining gas : overall, they're running out of the molecular gas (the stuff in the engine itself actually doing the business of star formation) but actually gaining atomic gas (the stuff in the tank). This is a small change for dwarf galaxies but really quite large (70 % !) for massive ones, even to the point where the galaxies have grown significantly in overall baryonic mass as a result of this. Galaxies then, are still assembling even today, not just from mergers but from the condensation of the thinnest gas in the intergalactic medium.

What this means is that something is changing star formation efficiency. Somehow galaxies in the early universe were able to efficiently convert all their gas into stars, whereas today something is preventing the HI from cooling into molecular gas which can form stars.

Why could this be ? Frustratingly they remain silent about this, which is annoying because this is such an obvious question you can't not ask it. Especially since modern galaxies are much more metal-rich, which enables very much faster cooling and condensation of the gas : if anything they should be more efficient at forming stars, not less. On the other hand there are far more stars around today (though less energetic), so perhaps stellar feedback is to blame.

Well, I dunno. Fair play to 'em for being clear about all their many assumptions, but where we go next with this is anyone's guess. It needs a lot more independent studies before anything else can be said about this. 

Dotting the i's and crossing out the dark matter

This paper revisits one of our old friends, those galaxies without dark matter.

When last I checked in on this, it seemed to be settled that indeed they do lack dark matter, and they can be explained by conventional (though rare) interactions which can strip the dark matter but leave behind a surviving stellar core. This is probably still the case. The whole distance debacle appears to have been decisively settled such that the low velocity dispersions of the objects are indeed consistent with little or no dark matter. The authors note in the introduction, however, that a few diehards maintain that maybe we're just seeing them close to face-on, which would hide the signatures of rotation.

Dedicated readers will recall that I myself thought this quite plausible for some other, similar objects, until a recent paper finally convinced me that this just isn't tenable. This is why it's important to check every hypotheses as carefully as possible. The only thing that ever really settles the arguments is better data.

So what the present authors have done is gone and get measurements of how fast the stars are moving in the notorious NGC 1052-DF4. Already the globular cluster data is clear that it can't possibly have dark matter, with a velocity dispersion of just 4 (!) km/s. But direct measurements of the stars in the galaxy itself would be much more decisive, because nobody's ever really going to be happy with data from just seven globular clusters.

And the measurements, if sufficiently precise, can help beyond settling the major issue. As they say, different formation scenarios (such as tidal stripping versus collisions) are expected to result in different amounts of dark matter remaining, though always of course on the low side. Determining just how low this is requires extremely precise data, the kind you can only get from the stars themselves.

So that's what they go and do. The have 14+ hours on the Keck telescope and find the measured velocity dispersion, though larger than from the globular clusters at 9 km/s, is still entirely consistent with the galaxy having no dark matter whatsoever, and inclination angle effects can be neglected. Making this measurement is a much harder task than in regular galaxies, because here the dispersion is so low that even motions of individual stars need to be properly accounted for. After various corrections, their final estimate of the true velocity dispersion is just 6 km/s. The traditional NFW profile for the dark matter for an object like this would give a mass about three orders of magnitude greater than what they observe.

(I confess to being a little caught-out here. The globular clusters are found further away than the stars but their velocity dispersion is lower...? So used to declining rotation curves am I that this seemed really weird ! Then I remembered this is exactly what's supposed to happen according to Kepler)

They also give a much better explanation as to why the globular cluster population of these objects is interesting in itself, compared to other papers which I find never get the point across. For normal galaxies there's a nice simple linear relation between total dark matter mass and number of globular clusters. These objects, according to that relation, are consistent with much more massive dark matter halos. That is, they have far more globular clusters than one would expect based on their small/zero mass of dark matter. They don't fit the standard models of galaxy formation at all. 

In fact they're outliers in at least three different ways : they have less dark matter than expected given their stellar mass; more globular clusters than expected for their stellar mass; and more massive individual globular clusters than is typical. And their globular clusters are, they say, "remarkably consistent in colour".

They don't here speculate much as to what all this means for the formation scenario, and in this case, given the controversy that has engulfed these objects, I can't say I blame them. They do however note that there is some tension with cored dark matter profiles as well as the standard NFW ones, though it's a rather weak tension. More interestingly, they say that the velocity dispersion is "clearly inconsistent with MOND", which predicts > 12 km/s. This is fun because previously claims of inconsistency were shown to be premature because they hadn't accounted for the external field effect, a MONDian effect whereby nearby galaxies can change each other's dynamics in a way that doesn't happen in standard gravitational models.

We shall see where that one goes in due course, I'm sure. Still there are many questions about these objects. Since there are two in the same group, the presumption is they must have formed the same way. And somehow they have to have survived in the group without being destroyed by tidal forces, which is counter-intuitive for large, low-density objects. So if the major issue is settled, it hardly feels like we've heard the last of these yet.

Is it a group ? Is it a cluster ? No, it's a supergroup !

This paper caught my eye for the wrong reasons, but it turned out to be interesting all the same.

It's time for another look at our old friends, Ultra Diffuse Galaxies : those faint, surprisingly large smudges which have been confusing us all for some years now. I've covered their dynamical masses umpteen times before, but their gas content is interesting not just in understanding their dark matter content, but for their star formation too. So when this study said it was using an HI survey to examine UDGs, I was hoping for more than one measly detection.

They start with a nice little overview of possible ways to form UDGs. They might be dwarfs which were spinning unusually fast, so distributing their gas over such a large area that its density was too low for much star formation. Or it might have been early star formation that drove the gas out to larger distances, with the effect being the same; likewise tidal encounters could do something similar. More dramatically, they might be as massive as Milky Way-sized galaxies, with something happening to quench their star formation in a process yet to be understood.

The authors are using the WALLABY survey on one of the SKA pathfinder telescopes, which gives decent resolution and good sensitivity over a very wide field of view. Their target is the "Eridanus Supergroup", which is fascinating in itself. Only a few such supergroups are known, and they're thought to be groups in the process of merging to form a full-on cluster. 

And they really do blur the boundaries between groups and clusters. One individual group in Eridanus has hot X-ray detected gas despite its small number of constituent galaxies. I'm going to have to update my introductory talks to include stuff like this – the presence of X-ray gas is a common way to distinguish groups from clusters, besides the more-obvious parameter of sheer number of galaxies. A small group which has this hot gas is not at all typical. Though it would have been nice of the authors to give a table describing the properties of the various Eridanus sub-groups; the maps that they show are very crude and not useful.

The X-ray component seems to be quite substantial, since galaxies here have measurable levels of HI deficiency. In clusters this would usually be interpreted as a classic signature of ram-pressure stripping. Yet with only a few galaxies, rather than a few tens or few hundreds as in most clusters, it's anyone's guess where this hot gas actually came from in the first place. They also note that there are two "enormous HI clouds without optical counterparts" in Eridanus, although that's not quite how I would describe them.

Anyway, they make a search for UDGs using the usual criteria and find 78 candidates in the WALLABY HI survey region. Since they don't have redshifts, most of these are probably background misidentifications but that's okay. But because this catalogue comes from something designed to search at greater distances, they apply an additional size constraint which leaves them with just 6 candidates.

Then they do something a bit strange : they search the HI cube using an algorithm. I mean that's fine, perfectly fine... but... it's not ideal in this case. Far better, I would think, would be to extract individual spectra of each UDG and see if there's any hint of a detection. After all, in this case the search is specifically for UDGs, so there's no need to insist the HI catalogue be homogenous – this is unnecessarily restrictive. Searching cubes blindly is all well and good but you have a much better chance of detecting the faintest stuff if you already know where to look. You want a blind search if you're specifically interested in which objects are gassy, not if you want to know how much gas each object has.

This all makes it a bit odd to call this a WALLABY paper since hardly any of the rest is concerned with the HI at all (they don't describe anything about their resulting HI catalogue, leaving me wondering if they only detect the UDG but that would be remarkable in itself). I mean they do try, but I think all the rest of the interesting stuff is in their discussion of optical relations, with the HI very much being an aside.

And there is interesting stuff here to be sure. They calculate the expected number of UDGs in each subgroup, which apparently follows a power law according to the group mass. They also show that (from other studies) the slope of this power law is invariant with environment, suggesting that environment doesn't matter much to the formation of UDGs. Which is very interesting considering that UDGs as a whole are a very diverse bunch, with some being blue and structured and some red and smooth (and a couple here, interestingly, which are blue and smooth).

While the numbers of predicted and candidate UDGs are consistent for two of the subgroups, for the one with X-ray gas there's a discrepancy : 17 predicted, none found. They suggest that in this presumably more mature group, most of the UDGs have already been disrupted or merged with the central galaxy. In that case though, I'd want to know where this power law comes from exactly, since most UDGs have so far been found in clusters : it seems a bit paradoxical to find the biggest disagreement in the group that's more like a cluster than the others ! 

And they also say this isn't a surprise anyway because of the number of candidate UDGs there is small, but this makes no sense to me at all. I cannot really get my head around it. The claim seems to be that they can predict the number using the mass of the group (an independent value), but that there's a discrepancy is because there's not as many found as they expected... huh ? The reason for the disagreement is that the numbers are different ? That feels like a truly weird tautologous statement. I may well have missed something. 

While they make a brave attempt to consider if tidal stripping or ram pressure was most likely responsible for the UDG formation, with six objects this is inevitably a forlorn hope. Though they do find that one of the candidates is at the end of a long stellar tail in a pair of interacting spiral galaxies. I'd have made a bigger deal out of this.

I can't escape the nagging feeling that this paper is doing all the right things in the wrong ways, or at least in the wrong context. A blind comparison of HI detections and UDG candidates is great if you have substantial samples of both. Examining the scaling relations of UDGs in different environments is useful when you have novel data sets to present. 

As it is, they have two interesting things : the detection of a single UDG in HI (showing that these are still very unusual) and a UDG at the end of a tail. Personally I would have concentrated heavily on the details of these two individual objects. Show me the HI spectrum ! Tell me more about how that near-tidal UDG compares to other UDGs ! Leave the scaling relations for when you have more things to relate : individual objects are still themselves worthy of study; not every study has to be about whole populations.

To find HI, first eat cookies

Just a quick one because this is well outside my field but I absolutely cannot ignore it.

This simply wonderful paper starts with the utterly brilliant name of "WTH! Wok the Hydrogen" and it continues in that vein for the whole paper. It's all about building a radio telescope for public outreach purposes out of old bits and bobs you can find lying around in your kitchen. Of course, the main feature is the wok itself, which serves as the dish. Is it a parabola ? Is it a sphere ? They're not quite sure, but suspect it's neither because woks aren't normally built with radio astronomers in mind.

In any case it doesn't matter. With a diameter of 60 cm, only a few times the 21 cm emission this thing can detect, it has a 24 degree beam. The photo of the telescope sitting in a plastic tub that you could find in any hardware shop is by far and away the best observing setup I've ever seen. With a beam that large, you really just have to aim it vaguely in the direction of the sky and it'll do its thing.

It's not just a wok and the plastic containers though. There's also the cookie box, used for shielding the electronics. Apparently this makes a big difference to the sensitivity, allowing them to observe from the bustling metropolis of Hong Kong and still get a detection despite the billion or so mobile phones and other radio nasties in the area. They say this shielding is what makes the difference compared to other amateur radio projects that have had to go to quieter sites for detections.

Incidentally the use of the wok specifically is not just because of its ready availability in Hong Kong, but because its shape (whatever exactly that might be) is better than a traditional satellite dish because it has a better focal length. And one further piece of household equipment they use is a microwave oven, which is a ready source of copper wire for the dipole antenna itself.

How well does it work ? Look, this thing is never going to detect a pulsar. But it can detect the HI 21 cm emission from the Milky Way in 10 minutes. Point it towards the galactic plane for 10 minutes and BAM, a detection results. Point it away for the same amount of time and BOOM the signal goes away. For public outreach that's brilliant. You can't really do any more than that; they say it's just not good enough for mapping the galaxy or measuring a rotation curve. But who the hell cares ? They've taken a bunch of old junk and turned it into a $150 radio telescope that can detect gas in the Milky Way. That's a missed opportunity for Scrapheap Challenge (Junkyard Wars for Americans) if ever there was one.

And the best bit ? Our institute director wants to build one. Next year's open day should be interesting indeed.

A pleasant shock

Last year I made a rare foray into investigating the scary world of astro-chemistry. Anything other than atomic hydrogen is generally too complicated for the likes of me, but I was co-author on Robert Minchin's paper looking at how ram pressure can excite the CII (carbon two) line. Urrgh !

Why would I venture into such horribly difficult territory ? Well, it turned out that this was a really neat and wholly unexpected way to validate our models of ram pressure stripping. When a galaxy moves through an external medium like the hot, low-density gas found in galaxy clusters, the pressure on its own gas builds up. This injects energy, and it turns out that this excites it to radiate in the CII spectral line. So by looking for galaxies with more CII than expected, we can work out if they're experiencing ram pressure or not. 

What's really neat about this is that it's a totally different way to search for this than the traditional direct hunt for streams of stripped gas. It gives a handle on what galaxies are experiencing even if those effects aren't strong enough to actually strip the gas, and remarkably, it actually seems to work pretty well.

Today's paper features Robert Minchin as a co-author and it expands the idea to a detailed study of an individual galaxy. While better statistics - i.e. more galaxies – would be nice, having such a well-resolved, in-depth look at a single galaxy is also an important way to check the basic premise of the model. As a disclaimer, while Robert and I go way I wasn't involved in this one at all. 

Strictly speaking it's not the CII itself which is the key signature, but the ratio of CII to far infra-red emission. FIR is dominated by stars, whereas CII emission, the theory says, should be caused by any source of excitation. So if a galaxy has an excessively high CII/FIR ratio, then you know something else is happening to it besides star formation. You have to use other clues to piece together just what that might be.

In this case the target is Arp 25, a beautifully lop-sided spiral galaxy in a group of about eight other galaxies, about 28 Mpc from us. It doesn't have any know stellar or gaseous streams associated with it, or even any especially prominent spiral arms, but when you look at it... it clearly ain't right. And the group itself, weirdly, has its own intra-group X-ray gas, something normally only found in much more massive clusters of tens or usually hundreds of galaxies. Not eight. Understanding how that happened is, I suspect, a whole project in itself, and this isn't covered here at all.

The authors use a whole suite of data sources, from optical to UV and X-ray to HI and, crucially, the ill-fated SOFIA flying telescope, which provides the all-important CII data. Most of these maps are very nicely resolved. And what they find is convincingly simple : there's a clear shock front with an enhanced CII/FIR ratio. The intragroup gas may be low density, but the infall velocity of the galaxy they estimate at over 1,000 km/s : still not enough to cause strong enough ram pressure to strip its gas, but quite definitely enough to cause CII excitation.

And in scaling relations too, everything fits. The shocked regions are very clear outliers in terms of having excessive star formation given their gas density, but they also have more CII than expected given their UV emission (a key tracer of star formation). And whereas normal galaxies have a clear, tight relation between CII/FIR as a function of FIR density, the shocked regions have an unmistakable excess. Again, everything fits, including some other relations I'm not even going to go into.

So this basic model really does seem to work very well indeed, including outside of clusters which ram pressure is normally associated with. The one sting in the tail is that people have been using CII as a proxy for star formation in high-redshift galaxies. This shows that things aren't so straightforward, that it really does seem to trace all sources of injected energy. But once you know about this, it's an asset, allowing us to explore the environmental effects on galaxies without having to look for the relatively extreme cases where the poor things are being absolutely clobbered. Now we can study them when they're only being lightly molested. Yay science !

Hidden in plain sight ?

Here's a fun paper that I need to talk to my globular cluster expert-friends about. The idea is that some of them are actually the remnants of satellite galaxies which have had their dark matter halos tidally disrupted. I get the strong impression that this isn't nonsense, but I don't know enough about globular clusters to judge exactly where it is on the credibility scale. But instinctively, I like the idea very much. Expect an update when I get the chance to check with those in the know.

First, the author notes that globular clusters show a linear relation between their total mass and that of their parent halo (i.e. the total dark and visible matter of their nearest galaxies), suggesting their formation is more related to the dark matter than stellar evolution. This extends across a huge range of sizes, so huge I'm a little worried... for example a globular cluster in one galaxy has a stellar mass of a thousand solar masses, whereas its parent galaxy has a total mass of about a billion. This smacks of the "big things are bigger" bias : of course you see trends if you extend things this far. You can't really avoid it. But so long as the relationship is sufficiently tight, it's still interesting.

She also says that whereas faint galaxies have just a few, old clusters, the bright galaxies and mergers have lots of young clusters, suggesting that they're destroyed over time. This could mean that some have different origins (some being primordial and long-lived, others more transient) as she suggests, but I don't see why it couldn't be that they all form in the same way and the surviving clusters in faint galaxies are just those lucky enough to have avoided destruction. I suppose this depends on how short the destruction timescale is though, and from what I can remember from innumerable meetings, I would guess it's expected to be short. So this is probably a fair point.

A really interesting detail she mentions is that some globular clusters are now known to have multiple stellar populations, that is, having formed their stars in bursts at different times. Which she says shouldn't be possible, because the first generation of stars ought to have strong winds and explosions that blast out all the remaining gas and prevent any further star formation. If, however, the clusters have plenty of dark matter, then their mass would be enough to prevent the gas from escaping, allowing it to fall back in and form more generations of stars.

This paper is only submitted, not yet accepted, and in my opinion it's still a little rough around the edges. I would have liked a bit more information about the simulation setup because as described it seems too good to be true. Following the formation of individual stars ?? How the hell does it manage that ? Is it hydro, n-body, what ? I mean, I could look up the cited papers, but I'm not going to. And some terms are used a bit inconsistently, e.g. "environment" is used both on the large scales of whole galaxies (e.g. whether they're in groups or clusters) but also to the situation of the star clusters (whethere they're in dwarf or giant galaxies). Which can be somewhat confusing.

Another rather intriguing point she notes from her earlier simulations is that the minimum mass a dark halo needs to allow star formation is environmentally dependent. This is counter-intuitive but it makes sense. If a massive halo starts forming stars, the chemical processing enhances the metallicity and the supernovae and winds can disperse this enriched gas into nearby halos. The higher metallicity allows it to cool more easily, making it less resistant to gravitational collapse and so more prone to forming stars. And I think that's pretty neat. Sometimes in my public talks I go on a protracted, deliberately-breathless rant about the complexities of star formation and feedback, but this just isn't something that's ever occurred to me.

Until recently I've thought of galaxies and star clusters as really qualitatively different, discrete objects, with only hints of a more continuous categorisation being given at the NAM conference in the summer. But the author says there are four basic classes forming an elegant sequence : pure dark halos, dark-matter dominated satellite galaxies which have some stars, stripped halos which are stellar-dominated but with some dark matter, and genuine star clusters which completely lack dark matter. This has a very strong instinctive appeal to be; intuitively it just makes sense that things shouldn't be so strictly one thing or another as in the classical picture.

And how the dark matter can be stripped from a small galaxy to turn it into a star cluster has in fact already been addressed in relation to much larger objects. In simulations, dark matter is structured in a spheroidal halo, with its particles in random, radial orbits. Unlike the disc, even particles which are sometimes found in the central regions can also sometimes be found much further away. So the dark matter is continuously vulnerable to tidal stripping, whereas the central stars and gas are much more resilient because they're always deep in the gravitational potential well.

Much of what follows is a detailed, quantitative examination of the simulations. The upshot is that it broadly seems to work : it's not perfect but the basics are there (and the imperfections seem to depend on missing physics not included in the algorithms). It even includes globular clusters with multiple stellar populations. And best of all, it includes the testable prediction that if this is what's really going on, we should see many more globular clusters around the most distant galaxies, those in the early universe.

Personally I think this idea is very intriguing. But there's an elephant in the room which is deafening by its absence : what the hell does this mean for the missing satellite problem ? Pure dark matter simulations predict way more galaxies than we actually observe – could it be that when you include the baryonic physics, most of them just dissolve ? That'd be extremely satisfying, much more so than trying to get the feedback precisely balanced so that just the right number happen to form stars. If in fact most of the globular clusters we see are the surviving satellites after all, this would both unify the studies of star clusters and galaxies and solve a major problem with the Standard Model of cosmology.

It's too good to be true, of course, and the author doesn't mention this at all. But I like it.

EDIT : After discussions with the experts, this paper definitely isn't nonsense. The idea that some globular clusters could be remnants of disrupted galaxies isn't new, so here the novelty appears to be that this can potentially explain all or a very large fraction of them. We noted that the objects explored here are all very small, typically a lot smaller than Milky Way globular clusters – this probably is only because of the use of dwarf galaxies in the simulations, and they do lie neatly on the scaling relations for much larger objects. That's only a minor caveat, but it does explain why the author doesn't look at the missing satellite problem as the objects being studied are just too small.

A potentially bigger issue is that all models apparently predict more globular clusters in the past, so more quantitative comparisons are needed to show how this can be distinguished from alternative ideas. And why all the objects in the simulations appear to lie below the line indicating they should be tidally disrupted, we're not sure. There seems to be a bimodality in the simulated objects : either they're dark matter dominated or have very little, which is not much discussed. Presumably this reflects the vulnerability of these objects to stripping : if they fall into a situation where tidal forces can remove the dark matter at all, then this will generally remove almost all of it, whereas if they don't, they won't – they won't lose any through internal processes. Finally, the experts mention that the big challenge for globular clusters is explaining anti-correlations seen in their chemical abundances, though that one's beyond my pay grade.

I've also made minor edits to the rest of the text and corrected my mistaken pronouns.

Kepler's Krazy Kurves

I'm going to hate myself for the title but meh.

This paper is about the rotation curve of the Milky Way. Of course we all know galaxy rotation curves are flat, and that's one of the biggest reasons we think the Universe is dominated by dark matter*. But even so, if we go far enough away (so that the great bulk of the mass is interior to a given position), rotation curves should eventually start to decline in the good old-fashioned Keplerian way.

* Well not really, but see below.

Now I'd thought that this was something that hadn't been previously seen in any galaxies, or if it was it would be one or two exceptional and/or highly uncertain cases with dodgy measurements. So given that this paper claims to have detected the Keplerian decline for the Milky Way, I was expecting them to make a big song and dance about this, possibly also cheerleading for either vindicating the Standard Model or for finding new MOND-compatible evidence. I'd have been happy to read it either way, but they don't do this at all.

Instead, the press releases* pick up on something I would tend to regard as fairly trivial : the exact mass of the Milky Way. Which is normally the astrophysical equivalent of a dick-measuring contest, pointless, petty, and greatly exaggerated.

* I do want to say that Brian Koberlein is one of the best science communicators out there, lest this should be interpreted in any way disparaging – as I usually am toward press releases.

And to some extent that's true. The poor authors go through an enormous amount of very careful work to measure the rotation curve to as great a distance as they possibly can, but it isn't at all clear what they get out of this that's new. It looks, from both their figures and text, like this result is all but confirming existing findings, with any differences seeming to be marginal at best. Sure, it's an important confirmation (we shouldn't underestimate the importance of replicability !), but it would have been a lot clearer if they'd just stated this. As it is, they extrapolate by a factor of four in distance to find a difference from previous findings of the total mass by a factor two. Ain't nobody caring if the Milky Way has one or two trillion solar masses of stuff. 

They make another couple of annoying, though much more minor, claims which could also easily have been clarified. They say they estimate a radius for the Milky Way which is almost twice as large as the standard 13.4 kpc, but don't say under what conditions : presumably they mean the distance at which they directly detect material in orbit. If so this is uninteresting, because nobody thought the galaxy had a sharp edge*. And they say that their measurements can help resolve the important argument about whether the Milky Way's dark matter halo has a central density spike ("cusp") or a flat "core", but don't actually investigate this at all.

* I may well be misreading what they're intending to claim here, but this could have been reworded very easily.

But they do implicitly make it very clear that yes, the Milky Way does have a dark halo. The rotation curves that they derive from all the individual components (stars, gas, dust, and separate components of the bulge and disc) do not come anywhere near close enough to explaining the orbital motions without dark matter. Sure, the curve is Keplerian, but it's still way faster than expected, even at the outermost edges, than from the baryons alone. This is an important reminder that really it's the speed of rotation, not the shape of the rotation curve, which determines whether there's dark matter present or not.

And I have a lot of praise for other aspects of the paper too. They note that one of the earliest flat rotation curves comes from 1925 (!), much earlier than I thought – though the figure itself shows a very high scatter, and nobody would hold this as in any way conclusive. Still, a good bit of archaeological research there. 

More importantly, while their Milky Way mass measurement may be consistent with other values found from similar methods, it's substantially lower than those from other techniques. Estimates made using extragalactic sources, like the motions of the Magellanic Clouds, have found masses up to five times greater than the present estimate. What they say this likely means is that these other methods are just not robust enough : these other components aren't in nice stable orbits, so of course their motions don't really reflect the true mass. And kudos to the authors for emphasising that our knowledge of our own Galaxy is still changing. That's what research is for, after all.

While it's disappointing that they don't venture any guesses as to what the Keplerian decline might mean for cosmology, this is probably a wise move. They do, however, give a nice overview of the (as I suspected) extremely limited and suspicious claims for other such detections in other galaxies, making it clear that the Milky Way is quite unique in this regard. And this points to a pet topic of mine, that those claiming Standard Model adherents have to point to the special nature of the Milky Way to avoid conflicts with theoretical predictions are absolutely right to do so. Sure, we should assume we're a typical spiral galaxy by default, and it's therefore right to be surprised when we find it does unusual things that typical spirals shouldn't do (like having satellite galaxies in a plane). But it's wholly wrong to insist that we must be typical even when measurements clearly show that we're not.

In this specific case they point to two possible interpretations. First, it might only be the unique nature of the methodology that gives us this Keplerian decline, and if we had comparable data for other galaxies we might well see the same thing. Second, perhaps more interestingly, the Milky Way's other unusual attributes (being a four-armed isolated spiral which hasn't experienced any recent major mergers) might indicate other types of galaxies we should examine to see if they too show the same thing. Maybe this declining rotation curve is a consequence of that, in which case, other similar galaxies should show the same feature. This makes things testable.

Finally, they note that the classic NFW profile often used to approximate the dark matter halo doesn't work in this case, as it can't predict a declining rotation curve (as you go further and further away, says NFW, you just keep enclosing more and more mass). The Einasto profile has no such problems so we should all use that instead.

What began for me as a mildly baffling disappointment turned out to be a really very nice piece of work. It doesn't have any direct bearing on the Standard Model, but it does suggest further evidence that our Milky Way is weird – and that the measurement technique, especially within our own Galaxy, really matters. It's that comparison with different techniques I think should have been emphasised much more in both the paper and the press releases. Without that, the whole main point of the paper is easily lost.

What do you do if you like a galaxy SO much ?

 ... you put a ring on it, obviously.

Sorry.

Anyway, today's paper is about one of the rarest sorts of galaxy morphology, the polar ring – that is, a ring orbiting at a high angle over the plane of the galaxy disc. If you've ever seen my introductory galaxy evolution course, you know that rings are very much a thing, but what I didn't mention was just how rare they are. Which is very. According to the authors, previous estimates put them at a frequency as low as about one in a thousand. 

Being incredibly rare either makes them automatically interesting or just unimportant, depending on your point of view. In the memorable words of Alan Dressler, polar ring galaxies "look odd and forced, like a dog wearing shoes". Personally, I would lean towards the discovery of any dog that voluntarily decided to wear shoes as being super interesting, even if learning all about such an animal wouldn't tell us much about the general doggy population.

How do they form ? Two main mechanisms : direct head-on collisions of two galaxies, and the result of warps in galactic discs. If it's the latter, they might just be extreme cases of warps that result from natural instabilities from the rotation. Either way they wouldn't last very long, but could tell us more about how galaxies evolve when left to their own devices or when they interact. It could also be that a much higher fraction of galaxies experience a ring at some point, and that we only see a very small fraction with rings because no individual structure lasts very long.

Polar rings are generally seen in stellar emission. The authors here found a couple of candidates using an HI survey, with the stellar component of one of the rings being very faint and not detected at all in the other. But these are two galaxies out of just two hundred in their sample, suggesting that maybe gaseous rings are an order of magnitude more common than stellar rings. Of course with such small sample sizes this is reaching a bit, but it's still an intriguing early result from the shiny new WALLABY survey.

Sadly they don't really speculate much on the origin of the rings in this case. The environment of the galaxies is barely mentioned, since their main concern is to just to establish that they are rings. Personally I think they overdo it; to me I'd be inclined to say, "yep, that's a ring", but they really go to town on this. They make model polar/inclined rings and transform them into mock observations to see how much the noise and beam shape would affect their detectability, with the end result that one candidate is more likely a PRG than not, and the other is a strong possibility (the major alternative being that it's just a warp in the disc) but needs better resolution. 

They also note that the appearance of the rings changes depending on resolution : at lower resolution, they're easier to see by looking at kinematic maps, whereas at higher resolution ordinary integrated flux maps show them more clearly. Once again data visualisation matters. And it's nice to see that they actually do a lot of their measurements using the virtual reality iDaVIE software; it deserves some serious use rather than being an entertaining gimmick.

All this means that we might be missing a significant number of ring galaxies, both from looking at the wrong wavelengths and making the wrong sort of maps. It's unlikely that rings are so numerous as to constitute a significant portion of the galaxy population, but in terms of understanding the rings themselves, this could be really important. One bit of speculation that they do indulge in is to note that the HI rings might be the progenitors of the stellar rings, or perhaps the two form by completely different mechanisms.

Predictably this ends in a classic case of "we need more data". But that's okay, because they establish good ways to identify rings and WALLABY will in fact provide that data. With WALLABY alone expecting to increase the number of HI detections into the hundreds of thousands, it's going to be interesting to see how this develops over the next few years.