Follow the reluctant adventures in the life of a Welsh astrophysicist sent around the world for some reason, wherein I photograph potatoes and destroy galaxies in the name of science. And don't forget about my website, www.rhysy.net



Wednesday, 27 July 2022

Attack Of The Multitudinous Space Blobs

Space blobs ! They're like regular blobs, except... they're from space...

I continue my long-standing policy of blogging every paper I'm involved with, but this latest one poses more of a challenge than most. In general, we try and write papers that focus on the science. Of course, you can't do science unless you've got something to do science on. In my case, this usually means finding the atomic hydrogen gas* component of galaxies. This is a subject about which I've gone on rants ad nauseum, albeit usually with lots and lots of pretty pictures.

* Most gifs in that post are broken, because Google FRICKIN SUCK at maintaining gifs for some reason. They're uploaded files, for heaven's sake, they shouldn't just stop working.

This post has to be a bit different. In this new paper, the balance is more towards, "look at all this lovely data" and away from" "here's what we learned from it". Let me explain why.

The main survey I get to play with is the Arecibo Galaxy Environment Survey, which looks at 16 different fields on the sky, each looking at different galaxy environments. In the field described in the latest paper, we found... rather a lot of detections, over 450 in fact. Only one other field so far contains more, and we haven't done much with that one because it was far too scary. 

Yes, exactly like sorting a big pile of clothes, except the clothes are galaxies, and if you put them in the wrong piles, a referee will shout at you. This analogy definitely works.

This large number means that doing an in-depth examination of such a complex field is just a right pain in the backside. And there's another, more pragmatic reason : it's not quite "publish or perish" for grant reporting purposes, but number of publications is a factor. So we could wait five years and get a bunch of papers all at once, or do one now and go from there. And lead author Boris Deshev was funded by a grant which was expiring. So we chose/were forced into the latter approach.

But actually this might be the better option. Science is not just a process of hypothesis-testing but of open-ended exploration and discovery. In fact that's one of the big advantages of AGES, that it has these regions we deliberately target but also a huge amount of foreground and background where we have no idea at all what we might find. That's where the excitement is. The downside, of course is that it's much harder to formulate a specific goal when you have no expectations. Sometimes you'll be lucky and a clear result will jump out at you, but often it's not easy to know what questions to ask. So it can be better to just say to the community, "look, here's the data, see what you can make of this."

Now you might be wondering if 450 sources is really such a large number. When you see the data, you'll probably understand why : even extracting those signals is a challenge, then we have to measure them, compare them with existing measurements, cross-correlate them with other catalogues, and then try and work out sensible ways to arrange the data so that we can squeeze valid result from it. It's not really 450, it's many thousands of different measurements and comparisons. Creating a sensible framework to organise everything isn't easy.

And samples of this size have their own challenges : they're small enough that every value matters, as even a few weird outliers can skew the trends (whereas with big data they'd get washed out) but large enough that taking minute care with every galaxy becomes burdensome.

Wait, what about the space blobs ?

Oh, sorry, I got a bit ahead of myself. Perhaps I should give some background overview of the science and the survey. If you're already familiar, skip ahead for the latest results.


Introduction

What's all this about hydrogen then ?

A good place to start, as atomic hydrogen is just about the simplest substance there is. One electron orbits one proton. We usually call it "HI" (H one) by convention. You can also have molecular hydrogen (H2, H two), which is two hydrogen atoms stuck together, or ionised hydrogen (HII, confusingly also pronounced as H two) which is just one proton. Or even maybe hydride (H-), but that one need not concern us further today because it's useless.

Hydrogen in general is the most common stuff in the Universe. All stars are ultimately made from hydrogen, but the steps to get there are complicated and involve some or all of its different versions.

HI is the most common variant of hydrogen. We think of it as the reservoir of fuel for star formation : this is where it all comes from initially. But it's pretty toasty, typically hanging out at around 10,000 K*. It cools very slowly, and this high temperature means it has extreme difficulty getting dense enough to collapse into stars : the temperature keeps pushing back. Once it does get sufficiently dense, however, it can cool to form much denser H2 (typically ~100 K), which doesn't have the temperature to resist a runaway gravitational collapse, leading ultimately to star formation. Stars in turn inject energy into the surviving gas, ionising it to produce hot HII, which can slowly cool to HI, and so on.

* No intuitive feeling for how hot this is ? No matter. Just wait a few years and let global warming sort it all out.

Sometimes we say that HI is like a galaxy's fuel tank, and it's an interesting question how far this analogy can be pushed. With a car, you can still do 70 mph even if your tank is one-quarter full, but you'll stop pretty quickly when you reach empty. Likewise with a galaxy, perhaps. If the HI is in the tank, it's the molecular gas which is in the engine. So losing even a considerable fraction of HI doesn't necessarily mean you directly affect the star formation rate. The relationship status for HI and star formation is very much set to "it's complicated."

In terms of hydrogen, a typical galaxy looks like this : a big halo of very low density gas, within which lies the main disc of HI. The density of the HI rises towards the inner regions, where it flattens and you get a mixture of HI, HII, and stars.

Physically, the dense disc has the same thickness ratio as a CD. The larger HI disc is only a little bit thicker, while the halo is spherical.


So how do you find all this gas then ?

It depends what you're looking for. The hottest gas (~1,000,000 K) emits at X-rays*, which needs space telescopes, while HII can be seen using ordinary optical telescopes. H2 is hard to see directly but there are ways of tracing it using other elements, often at mm wavelengths. HI is comparatively simple. Very helpfully, it spontaneously sends out radio waves of 21cm wavelength, so all you need is a radio telescope. You can detect it directly and it's super simple to convert between measured brightness and physical mass. 

* This is ionised, but not usually referred to as HII. As I understand it, HII is used somewhat interchangeably with the recombination lines that are used to detect it. For this to happen the gas has to be only just above the temperature where ionisation happens. Too hot, as in X-ray emitting gas, and no recombination happens so you can't detect it at optical wavelengths.

The good thing is that the required radio telescopes are not too difficult to make. They don't need especially complex electronics or other equipment, they just need to be - and this is the downside - friggin' massive.

"It's what you do with it that counts !" said no astronomer ever. Well, not really, but sometimes bigger really is better.

Once you've got the data, you go through it by eye looking for the signals, something I've covered before. And in our data the galaxies aren't well resolved, so they all look like blobs. In this case, lots of blobs.


Ahh, I see. And galaxy environments ?

Nothing to do with saving the rainforest or tying oneself to trains. Rather, galaxies don't all live in the same places. Some live in very sparse voids where there's not much of anything. Others live in little groups of a few other galaxies orbiting each other, while others live in massive, rich clusters thousands strong. And on the largest of scales, groups and clusters alike are mostly found in giant filaments.  

Most galaxies live in small groups, with probably only ~1-2% in rich clusters. This means we have to be very careful with drawing general conclusions about galaxy evolution from such extreme locations, but they do let us sample huge numbers of galaxies with only a small amount of observing time.


Does gas vary with environment ?

Indeed. Remember that the HI is generally more extended than the stellar disc of a galaxy. This means it's less tightly bound, so any process which disturbs the galaxy (like other galaxies passing by, cruelty to animals, people who wear socks with sandals... that sort of thing) has more of an effect of the gas than the stars. So by looking at the gas, we can get a better idea of what the different environments really do.

AGES was designed to cover the whole range of galaxy environments, from voids to isolated galaxies to small groups and giant clusters. But each of our target regions also covers what's in front and behind of its target as well, so we get an awful lot of bonus data. This "AGES volume" is actually where most of our detections are found. And that, more than the target cluster, is what we started with in this paper.


AGES XI : The Franchise Continues

We're not going to let a little thing like the collapse of the telescope stop us, are we ? Of course not. In this paper we look at the Abell 1367 cluster. It's not a sexy, famous cluster like Virgo or Coma, but like all clusters, there's plenty of interesting stuff going on here. In fact it contains one of my favourite objects of all, the appalling-named "blue infalling group" :

The brightest blobs are galaxies, with the faint trails between them being ionised gas. Image from this paper.

It always makes me think of the smoke trail from a bumblebee with its bum on fire. What's actually going on, we think, is that a group of galaxies are orbiting each other while also falling into the cluster proper. As they enter, ram pressure stripping removes their gas, which traces out these nice helical patterns. Try turning on a hosepipe and waving it around and you'll see what I mean, and possibly soak the neighbours too. Don't worry, it's very hot, so they'll definitely thank you.

Or, the internet bizarrely lacking adequate hosepipe gifs, you can use ropes, but that's not as much fun.

Add in this particularly weird hot gas cloud and some very long HI tails and the cluster begins to seem like a place worth studying. Actually, the very first paper I was on (my role was pretty much 100% doing observations) was looking at the earlier AGES data of this region, which covered the central 5x1 degree strip. This covered the main part of the cluster but not much else. Interestingly, we saw that the galaxies detected in HI didn't follow the general pattern of galaxies detected through more familiar visible-light surveys. In 3D space they're distributed like so :

Galaxies detected with gas are in black, while those without are in red.

What's going on here ? All the optically detected galaxies are in a neat line ! This is the finger-of-god effect. But look at the axes : we have velocity, not distance. Now as a proxy for distance, velocity (which is much easier to measure) works pretty well in general. This is Hubble's famous law, that there's a neat, linear relation between the two. But though this works just fine in most places, this isn't true in massive clusters*. Here the great mass of the cluster means that galaxies are moving much faster than in the general field, even though they're all actually at the same distance.

* In fact, as early as 1932, Fritz Zwicky found that cluster galaxies don't follow Hubble's Law - which constitutes some of the earliest evidence for dark matter.

Okay, so the optical galaxies look like a "finger" because we've measured their velocity and not true distance. If we could measure distance, we'd probably find they were all much more bunched-up here.

From another, much larger, survey. On the left, velocity is used for distance, hence all the fingery "streaks". On the right "true" distance is used, or at least a better approximation of it. No streaks !

But the HI detections don't show this behaviour. They seem to be found all over place, as though the cluster hadn't made any difference. Why not ? Are gas-rich galaxies like honey badgers and just don't care ? Nothing so exciting I'm afraid; it's just a selection effect.

Galaxies in the centres of clusters can easily lose all their gas as the clusters' own hot gas pushes it out and ionises it. At this distance (92 Mpc, 300 million light years) even a slight loss of the HI would make it undetectable. So we can only detect galaxies which haven't been much affected by the cluster - our detections are likely either just on the outskirts and entering it for the very first time, or not quite at the same distance after all.

Does this mean there's no point in our survey ? Is even the mighty Arecibo inadequate to the task of detecting gassy cluster galaxies ? This is too pessimistic. Rather, we can still detect galaxies just entering the cluster, and that's important. Understanding whether galaxy evolution is dominated by clusters or if they can have equally thrilling adventures elsewhere is controversial. The Blue Infalling Group is a nice example, but doesn't by itself prove that this so-called "pre-processing" is very important in the grand scheme of things.


What did we find this time ?

Space blobs

Well, now the survey is complete we have the full 5x4 degrees of coverage, so we can look in much more detail at those galaxies on the cluster outskirts. But first, here's what all those blobs actually look like in the original data.

Not my prettiest picture, to be sure... but you can play with this interactively online, which is quite fun. Might take a minute or two to load.

I told you going from the data to a catalogue wasn't easy ! We use a combination of manually looking at the data and automatic source-finding algorithms to turn a bunch of blobs into a list of carefully-measured galaxies. In this case, because the galaxies are preferentially found over narrow velocity ranges, a lot of detections are confused with each other and hard to disentangle. Cataloguing involves a great deal of careful work to differentiate between galaxies as best we can, and flagging those where it's just an ugly unsalvageable mess. And large swathes of the data are dominated by whacking great big blobs, which are not alien megastructures but the effect of interference (yes, sometimes the hunt for aliens really does turn into a hunt for washing machines and other annoying electronic equipment).

This picture of the blobs is in the raw pixel coordinates of the data. When we convert everything to proper physical units, we see our our detections have quite a different distribution to the first paper :

Left : original, as above. Centre : only the HI detections in the original survey area (a few more are present now thanks to improved cataloguing). Right : the HI detections from the complete survey, over the region corresponding to the cluster (the full velocity range of the data is -2,000 -> +20,000 km/s).

It seems that there's a large group close to the main cluster which is rather gas rich, compared to the cluster itself which you'd never guess was there from the HI data alone. You can play with a 3D version of this here.


Silent but deadly : galaxies have gas we can't detect

So the environment of the cluster itself is different to what we thought. Boris has a thing against treating clusters as spherical systems, because they generally aren't. Often we see them still assembling, sometimes along filaments. Alas, the techniques used previously didn't really show much in this case - there's no evidence that galaxies experiencing anything particularly location-specific here. We were hoping that maybe we'd some clear signs that the environment varied : maybe the ones in that group would be especially gas rich, maybe the ones nearest the cluster would be gas poor, but actually no, not really.

Which is not to say that the cluster doesn't do anything. On the contrary, it's annoyingly effective at gas removal. For comparison, in the much closer Virgo cluster, we could detect galaxies with only 10% or even 1% of the gas content of similar galaxies found elsewhere. But here, as soon as galaxies lose even quite a small fraction (say 50%) of their gas, they become undetectable. And this means we can't properly comment on exactly where or which structures galaxies in the cluster are losing gas, because we just can't find the gas-poor galaxies. 

But it does enable a neat trick. In Virgo, we tried to "stack" the non-detected galaxies (adding their signals together) to increase our sensitivity. Which it did, by a factor 10... but no detection resulted. At the closer distance of Virgo, AGES is sensitive enough that it can detect the faintest whiff of gas*, so if it's not detected straight away, chances are it doesn't have any gas at all. Not so in A1367. Here, stacking results in a lovely clear detection :

* Do your own joke.

Stacking three different parts of the data set. As a sanity check, the blue curves show what happens when we stack galaxies we already detected - we always get a nice clear signal. The red curves show the stacked non-detections : we get a signal (a big bump) in the cluster and background, but not the foreground. The grey is a control stack where no galaxies are expected to be present.

Does this point to a difference between the clusters ? Probably not. More likely, our stacked sample does include some galaxies which are totally gasless, but it also includes plenty which are only slightly gas-deficient : the sort we could detect directly in Virgo, but can't here because of the greater distance. The two clusters may or may not be more effective in gas removal, but we honestly just can't tell. 

In short, in Virgo we'd already detected all the galaxies with even small amounts of gas. Stacking the rest didn't help because they'd lost the whole lot. Here, we can't tell which ones still have most of their gas, so we're stacking some which have no gas at all along with others which actually still have quite a lot left. Likewise in the foreground region here : at lower distances we're back to the same situation as Virgo... if we don't directly detect a galaxy here, it probably has no gas at all, so stacking doesn't do anything.


Is there a best way to measure gas loss ?

Getting this paper through the reviewing process was unfortunately another unnecessarily tiring exercise. Some of the comments were extremely valuable, but some smacked of the referee trying to write the paper for us, and other comments were honestly just wrong (somewhat amusingly, citing papers which actually claimed the exact opposite of what they said they did). And we made mistakes too, which made the experience about as much fun as wrestling a blindfolded bear while blindfolded ourselves.

What we did show, I think (though this one had to be heavily toned-down to make it palatable to the referee), was that we can use a simple statistical parameter to understand the effects of gas loss. Traditionally, we quantify how much gas a galaxy in a group or cluster has lost by comparing it with a control sample of similar galaxies in isolation. This "HI deficiency" can be computed for individual galaxies. It isn't at all exact though, because there's a lot of intrinsic variation in this parameter. Realistically, it can only tell you if a galaxy has :

  • A lot more gas than expected (very rare !)
  • A bit more gas than expected
  • About as much gas as expected
  • A bit less gas than expected
  • A lot less gas than expected

And that's it. You can measure it as precisely as you like, but you if pretend this precision is actually meaningful, you're fooling yourself. Still, it can be done on a galaxy-by-galaxy basis, which is very useful.

But with a sample like this it doesn't really tell us very much. As we've seen, even a slight gas loss makes the galaxies undetectable. We can compute a lower limit on the deficiency, but this is misleading, as the galaxies could easily have lost a lot more than that.

A better approach might be to just calculate the fraction of galaxies detected in HI. This can only be done on a whole population, so we lose all the individual information. And the choice of which population to select is arbitrary. But, for example, we can say that the detected fraction varies radially, being lowest in the centre (the main cluster) and highest in the outskirts (the first infallers). And because it's just a fraction, it doesn't give any impression of the gas loss being any particular amount, just that some has happened. Which is sometimes all you really need.

Now the referee got a bit over-zealous here. We did not say, at any point, that the detection fraction was a better parameter than deficiency, nor did we even ever give that impression (a better approach is not the same thing at all !). It's better in some ways, in some circumstances, but the converse is equally true. In fact the deficiency is always preferable whenever you can use it, but the detected fraction is simpler and always available (though it becomes meaningless if your sample is small). 


Brief intermission in which I rant against referees


I don't know why this happens, but it seems common for referees to think even weak, throwaway assertions in a paper must be held as inviolable Gold Standard claims that will survive any assault short of a tactical nuclear weapon. In my view, that's not what papers are for, never has been and never should be. All claims in papers are still subject to the broader peer review of the entire community. Papers are not textbooks but part of the discussion themselves; yes, they should be more rigorous than, say, blog posts (ahem), but there's no value in making them utterly unassailable. First, it can't be done (no referee is perfect), and second, because of that, it artificially shuts down discussion and so means that neither author nor community ever learn anything. Even papers are, ultimately, a step in the road - you want to make sure that potholes are minimised, but if you never put down any tarmac at all because the mixture isn't perfect, you'll never have a damn road at all


And now back to our feature presentation

Fortunately, what did survive the referee's onslaught (and I have to say was improved by it) was a comparison between deficiency and fraction as a function of local and global density :

Wait ! Don't run away ! It looks a bit scary, and it is a complex plot. So let's break it down into manageable chunks. First, the data is divided into two samples which we refer to as the global density : those in the velocity range of the cluster, and those elsewhere. The latter are largely in low-density regions, mainly groups in filaments. The former include galaxies in the cluster outskirts as well as deep in the interior. In the upper section we plot trends using deficiency while on the lower panel we use the non-detected fraction.

Let's forget about that nasty plot for a moment and just look at the first two panels. These are the most important anyway.

What we're plotting here is how the deficiency and non-detected fraction vary as a function of local density : that is, how many galaxies are present in different spatial bins. In an earlier version of the paper, we made a mistake and found that local density made absolutely no difference and that it was due entirely to global density (cluster or non-cluster). But when we corrected that, we found the opposite : there's a nice trend with local density, and clusters are only different because the local density is generally higher there. This fits with our other recent findings too.

(That we were able to spin some quite elaborate but plausible-sounding justifications for the first (wrong) interpretation I take as an important lesson in the dangers of rationalising. Again, no one paper should be held as anything more than evidence, never as proof.)

We can see from the plot that this result is much clearer with the non-detected fraction than with deficiency. The referee was concerned that this might be a statistical effect rather than representing real physics : yes, there's a clear trend, but maybe this is only because galaxies vary in some other way with local density. So to exactly reverse my previous rant, we wouldn't have looked at this without the referee's diligent nit-picking. Thank you, kindly pedantic stranger !

This is where the other panels come in : plotting deficiency/fraction as functions of stellar mass and star formation rate. We also took great pains to ensure that, unlike previous claims, we used a properly complete sample for this area, meaning that we're comparing like-for-like, something the referee seemed not to really believe for some reason.

What do we find ? Something like this :

  1. Overall, for all parameters, both deficiency and non-detected fraction are higher for cluster galaxies than for non-cluster galaxies. That's a good sanity check : there's definitely gas loss in the cluster, as we'd expect.
  2. There's little or no variation with gas loss (by either parameter) as a function of stellar mass. So our trends in gas loss aren't being driven by selectively detecting smaller or larger galaxies. This is important, because we'd only be able to detect high deficiencies for the biggest galaxies; likewise, we wouldn't expect to detect small galaxies unless they were actually gas rich.
  3. There's not much trend in deficiency with star formation rate. This might be a bit more surprising, but remember the fuel tank analogy : the relation between HI and star formation is indirect. But it could also be something simpler, since the error bars here are very large.
  4. In contrast, there's a clear trend between fraction and star formation rate. Here the error bars are much smaller. So tentatively, it looks like the large errors on deficiency are masking the trend : less gas, less stars. The detected fraction might be offering us more information in this case, at the cost of knowing which particular galaxies have lost gas.


Conclusions and where we go next

There are three main outcomes from this paper. First is the catalogue itself, together with an "atlas" : a set of visual charts for each galaxy, including the HI spectra and maps and the corresponding optical images, all labelled with the known galaxies present. Hopefully we can use this retroactively on our other data, looking towards the time when we release the full AGES data set for the whole survey.

Second, stacking galaxies works in this field. Having spent feckin' ages trying this during my PhD (there's a subheading in one chapter, "Four hundred million non-detections"), I was despondent that it would ever work at all. But it very clearly does, opening up scope for utilising this elsewhere. Boris has some intriguing ideas about what we might do with this.

Third, detected fraction can offer a viable alternative to HI deficiency. Despite the referee's protestations I think this could indeed be more useful at larger distances (where it seems galaxies were considerably more gas rich). Note that you don't get something for nothing - you get significantly reduced errors, but only because this has to be applied to galaxies en masse; individual information is lost. Fully understanding this parameter is more subtle than it may first appear, and needs a lot of work to ensure it is indeed telling us what we think it's telling us. It is absolutely legitimate that one can still have reservations about this. We think it's at the point where it deserves wider attention, but no more than this.

And scientifically, the view that it's local density which drives gas loss is an interesting one. It's not what one would naively expect. In massive clusters, gas loss is mainly due to ram pressure stripping, whereas in small groups it's from tidal encounters. These two processes scale completely differently, but we're seeing a smooth change in gas loss as a function of density. And in our previous study, we found that there's a smooth change with ram pressure as well, extending down even to very low pressures indeed.

This is a bit strange. It's a bit like saying that the fastest horse is always the biggest one : surely, you wouldn't expect a shire horse to come top of the league. You expect a broad correlation (Shetland ponies can't compete with a thoroughbred), but not a nice, simple, continuous trend.

It's hard to say what might be going on. My guess is the data isn't precise enough, that we're smoothing out any more sudden changes because the error bars are quite large : the change isn't really as continuous as it looks. I am not sure if this is a matter of getting better data or analysing the data in different ways to reveal if there really is any qualitative difference between small groups of massive clusters... then again, maybe the counteracting effects cancel out and the smooth change is correct after all. It's possible.


What next ? Well, some of our galaxies have no optical counterparts, for which we have follow-up observations using the Chinese FAST telescope. If any of these are detected that's an automatically interesting result. And of course, we can dig down deeper into the data to study the cluster itself in much more detail, as well as other individual objects with HI streams, galaxies with low star formation rates but high gas contents and other weird oddballs.... a veritable host of things ! As I said, defining the problem is likely to be the hardest part. But now we have a basic catalogue to start from, making comparisons and looking for trends gets a whole lot easier.

And all this from a bunch of blobs.

Tuesday, 12 July 2022

Expedition Cardiff II : The Fluffening

Last September I broke an 18-month stint in Prague by finally returning to the Welsh motherlands. Getting there was, of course, more of a procedure than usual. Since then the world has changed and changed again; in December I went back, but once more the travel situation is unrecognisable. I even had a visitor come to me from the once plague-ravaged UK, instead of the other way around !

So why should I bore you with yet another post about visiting Cardiff ? Well, this time the travel itself is part of the experience, not because of any so-last-year "pandemic" restrictions*, but because we decided not to fly and to take the dogs - a feat that would surely test the mettle of even Michael Palin. We planned this for two years, but the bloody resurging virus kept making it impossible. Until now.

* I use the sarcasm quotes out of pure facetiousness, not any retarded** anti-lockdown nonsense. That said, some caveats are to be found herein.
** That's probably offensive, sorry. But I cannot find another phrase which adequately conveys my disgust for the painful, gut-wrenchingly stupid libertarian nonsense about just letting everyone die instead of asking them to follow simple, common-sense guidance.

We wouldn't dream of trying this crazy adventure in a single stage. It would be cruel to the dogs and Shirley would miss a perfectly good chance for a visit home. So we made it a multi-stage process : a 14 hour train trip from Prague to the Hague*, four of five days there, an eight hour ferry crossing from Hook van Holland (which is practically in the Hague itself) to Harwich, and a five hour train trip to the fabled city of mighty Cardiff. Of course, we would have to do the whole thing again on the return journey. This is necessary because it's very hard to take dogs into the UK except by boat. So a boat out means a boat back.

* I don't know what it is with the Dutch and "the"s. The Netherlands, The Hague... it's all a bit the odd.

The dogs have done long train trips before. They're quite content in their little backpack carry-cases (they will voluntarily take themselves in), especially when equipped with cooling pads. This leads to many adorable photo opps.

Lulu (right) is the big sister, aged 5. Gilly (left) is our little pandemic pup, aged two.

Nothing interesting happened on the train. It's a scenic route, especially at the Czech-German border, though the last few hours we had no air conditioning. This was made all the worse because Germany, unlike everyone else, still insists on mask-wearing on trains. 

To be honest, rationally or not, I'm fed up with this. It's hard to describe. When I sit down and think about it, it makes sense. But I cannot honestly say I want any restrictions any more, I cannot welcome them as a reassuring safety measure. Something inside me has flipped, and while I might consciously advocate for keeping case numbers low, and certainly I follow all the rules, I cannot say I feel protected by the measures any more. The emotion is drained, possibly as a result of the other global crises*.

* You know the one I mean. I'm almost at the point of ignoring that one too, a case partly of crisis-fatigue but also an abundance of evidence that a wider global conflict is not at all what Russia is after. It seems to me that Russia has backed down too many times in the face of Western counter-threats to see them as terribly scary : concerning, certainly, but not a direct military threat to the whole of Europe. I will stress that my support for Western sanctions and indirect military support to Ukraine is 100% as strong as ever.

But I digress. While in the Netherlands, the main event was Taking The Dogs To The Beach. Lu especially loves the beach, and will start accelerating towards the sand as soon as it's in sight. They don't especially enjoy swimming, though their freely-floating fluff looks extremely cute when they're fully immersed, but they are increasingly appreciative of a good paddle. Lu did manage a little swim, until the six-inch high "waves" became too scary and I had to carry her across part of the flow. And watching them run in slow motion just never gets old.



The Hague beaches remind me a fair bit of the British seaside, except that they're a lot nicer.

View from a restaurant where the fish is so fresh you can almost see the boat it came in on.

Of course we also had time with Shirley's massive extended family and ate plenty of delicious food. I was finally introduced to Dutch pancakes, which are somewhere between the British and American styles, closer to the former. They are excellent. if you haven't had any, you should. And get yourself some kibbeling too, which - as I've probably mentioned before - is something that should be available on every street corner in the world.

Behold Gilly, the dog who rides elephants.

Then it was time for the signature feature of the trip, the overnight ferry. Deliberately slowing the crossing from the 5 hour day version to 8 hours at night ensures the boat both leaves and arrives at a reasonable time. And though expensive, you get what you pay for. First the terminal, which was a place so empty it would make a hermit feel lonely :

All the staff were extremely friendly from start to finish. Boarding with dogs is easy and you can take them in-cabin if you book this ahead of time (the only extra procedure being that you have to get them wormed by a vet a few days ahead of departure). 

The ferry itself was really very nice indeed. It has several restaurants, a sun deck, a literal poop deck* for pets, a basketball cage**, and even a tiny cinema and a casino. The cabins are cosy rather than small : spick and span, very comfortable beds, a giant porthole and a sizeable (and excellent) shower. The soothing motion of the boat and ability to lie properly flat ensured that I got a full night's sleep, something I wouldn't be able to manage on a plane unless shot by a tranquiliser dart. Watching the ships pass in the night all lit up, and waking up to the sun on a calm and silent sea, is at least fifty-eight times preferable to being on a cramped aircraft with its weirdly low-pressure, noisy, desiccated air and cramped seats. If you have the option to take the ferry, I highly recommend it : unlike an aeroplane, you come out of the trip feeling refreshed instead of drained. This is a slower but objectively better way to travel.

* Not its actual term, but this is what I shall call it.
** This is a cage containing a basketball court, not just a basketball.

And it's so damn shiny !


Arriving in Harwich we saw David Attenborough, or at least his very own personal boat :

A.k.a. Boaty McBoatface.

We proceeded to Manningtree and took an hour to allow the dogs some respite. They'd been quite unsettled on the boat until we took them to the poop deck - they didn't do anything, but seemed finally to understand that they were on a boat and therefore all the weird noises and vibrations became less scary. At Manningtree we had tea and cider at 8:30 in the morning, at which the sever batted not an eyelid nor showed the merest flicker of judgemental disdain. 

As it happened we arrived on the week of the train strikes the one day the strikes weren't happening. Consequently the only real difference was that we didn't have seat reservations, but we didn't have problems getting seats so this had no impact whatsoever.

When we got to Cardiff we immediately rejoiced in the marvellous beacon of hope and glory that is the United Kingdom by having fish and chips. Okay, the fish is not as good as kibbeling, of course, but the chips are unquestioningly the best in world. However I did not have long to revel in my patriotism, as we soon witnessed an auspicious sight :

At least the Welsh flags were standing tall. For them to have collapsed at Cardiff Castle would be like finding a dead raven at the Tower of London.

Thankfully the end of the trip was to prove far more restorative to whatever dull flame of national pride still lingers in my bitter and disillusioned soul.

The dogs, of course, were the stars of the show for the next two weeks. Papillions are virtually unknown in Britain, but given how people flocked to them like moths to a flame, perhaps that won't be the case on our next visit. It's interesting to see how much friendlier people are when you have a couple of cute bundles of floof in tow, but even so, I swear people have gotten nicer. I have this very cool T-shirt of a cartoon axolotl reading a book, with the caption, "reads-a-lotl". Hilarious, but I didn't expect a small boy in a train station to spontaneously point and shout "Axolotl readaslotl !!!" with obvious delight. What happened to the sullen, depressed, reserved countrymen I grew up with ? Weird.

Anyway, here are some pictures of the floofers doing what they do best.



Anyone who says I'm anthropormorphising by saying Lu is clearly thinking, "da fuq are you wearing" is, quite simply, wrong.

We've been trying since day one to get them to play with the same toy at the same time and IT FINALLY HAPPENED.

Non-dog activities included an open-air theatre production of Blackadder Goes Forth. This was hilarious, with the cast being superb : Baldrick was as close to Tony Robinson as you can get without being Tony Robinson, while the glorious, furiously insane portrayal of General Melchett and the unstoppably randy Lord Flashheart were both outstanding. There was some nice interaction between rain-soaked cast and safely-under-cover audience without feeling like a pantomime.

This being Wales, we had to visit a castle or we'd have been murdered by Plaid Cymru smothering us in our sleep with a sheep. We've done Cardiff* before, so this time it was Caerphilly.

* I've done all the local things before, but Shirley hasn't.




Caerphilly is a fine castle in a good location, with a tower that famously leans more than the more notorious Pisa and a set of replica siege engines that have been used in many a historical documentary. But its presentation to visitors does veer a little on the silly side, with a large dragon pit that has a somewhat ridiculous, over-lengthy and over-dramatic narration. It's kindof fun, but maybe overdoing it for the children ? I dunno, a better visitor centre and a café would work wonders. More ambitiously, Wales is in desperate need of a Hollywood blockbuster to present itself to the world. Not for nothing does it have monumental castles in dramatic locations, but we've done a hugely terrible job of selling Welsh heritage to the wider world.

Also noteworthy here was that Gilly chased a goose. Gilly is 2 kg of pure fluff and has the hunting instincts of a brick, whereas geese are several kilos more of angry beak and claw. Quite why the goose felt the need to run away I will never understand : being attacked by Gilly is like being savaged by a bag of candy floss.

We also did something I've never done before and walked over the Cardiff bay barrage to Penarth, somewhere I've not been in probably 30 years. It's quite nice.



Another blast-from-the-childhood-past was a visit to the St Fagan's Museum of Welsh Life. This is a collection of Welsh buildings reassembled from their original locations, including a farmhouse from 1610 and a medieval church restored to its original colour scheme.

The red colour supposedly protects against evil spirits. This wasn't applied to the privy, which was presumably defended by a foul stench.


The church is dedicated to St Teilo, who sounds worth looking up. Apparently he defeated a dragon he found in an orchard by picking it up by its tail and swinging it into a river, thus becoming the patron saint of cider. Or something like that.

Sadly I don't have any good photos but many of the paintings are meme worthy, including one guy who looks like he's crying because he's poked himself in the eye for literally no reason at all.

These days the museum also has more standard exhibit halls, which are worth a look around. If you ever want a demonstration of the difference between the Welsh and the English (we sometimes say that the Welsh define themselves by being not English, but there are real differences in the cultures), look to the responses visitors have left about Thatcher's famous line about there being "no such thing of society". This includes such things as "murderous bitch" and "you stole my father's bones", whereas the responses supposedly agreeing with the statement are far fewer in number and include such astute observations as "I like dinosaurs."

There was of course much pub and day drinking down the bay, with a visit to Cadwallader's ice cream café being a necessity, as was Waterstones. I got a lot of reading done in this trip, which means by backlog of things to blog is now even longer. I call that a success. There was also a food festival, and I'll likely be changing my profile photo as a result.


The end of the trip proved doubly rewarding. First, I got by far and away the most positive first referee report to a paper I've ever had (thank you, kindly and well-timed constructive reviewer !). The last paper I co-authored (to be blogged here in the near future) was also accepted just before the holiday started, but that had the usual protracted back-and-forth : nice to finally resolve it, but that doesn't really undo the months of mostly unnecessary wrangling. So to finally get a reviewer who just likes the paper from the outset is... well, for once I don't have to go on a rant about the need to reform peer review, and that, frankly, is just bloody great.

Second, if we'd waited two years for this trip, we'd waited even longer for this wonderful moment at the end :

I think I can look at this for some time to come and get a happy.

We watched the full news coverage of this for two days straight. After having endured this lying, contemptible, delusional fascist scumbag grind the country's reputation into the dirt for far too long, the release was almost cathartic - tempered only by uncertainty about who comes next*. Perhaps auspiciously, that same day the conservatory window spontaneously shattered and the washing line broke. I'm fairly sure this is the doings of the ghosts of my Conservative grandparents, although a significant caveat is that Boris Johnsons is to the Conservative party what a circus lion tamer is to serious wildlife preservation efforts.

* I've thought about writing a scathing political obituary to the racist piece of self-serving egomania, but I've decided not to bother. You may recall my last protracted anti-Boris rant ended with "fuck off Boris" and off he has jolly well fucked, so I'm satisfied. Defending a sex offender was, it seems, the straw that broke the camel's back, and there is scarcely a need for me to shout loudly from the rooftops, "I TOLD YOU SO YOU SODDING MORONS !" although I'd very much like to.

And then we did the whole thing in reverse. This time the dogs immediately accepted the ferry as their temporary home; we spent only a day in the Netherlands before catching the again miraculously smooth series of trains back to Prague. 

Well, there you have it. Cardiff remains a lovely place despite the current assorted crises (as well as its stupid 20 mph speed limits), and finally being able to show everyone the marvel that is the papillion was as good as I hoped. I re-iterate that slowing things down, taking the ferry instead of the plane, is just incomparably nicer. But now it's time to get back to science.

Thursday, 2 June 2022

My Chemical Enviromance

Yay, science post !

It's always nice when you wake up and see an email saying, "Your paper has now been accepted" but you're still half-asleep so you wonder "Dafuq ? I haven't written any papers, bloody spam journals again" but you're curious enough to keep reading the email just in case and realise "Ooooohh yeah, I didn't write that one but I'm a co-author, yay for me cos I didn't have to do any real work this time !" and then you fall back into a satisfying slumber.

Obviously we all sleep at our desks the whole time.

Funnily enough, that's more or less what happened to me last week.

This paper is by long-term collaborator and all-round thoroughly good egg Robert Minchin. It's all about my second favourite topic in my first favourite place : that is, it's about how galaxies lose gas in the Virgo Cluster (my favourite topic being little gas clouds that don't do anything).

I've covered gas loss in Virgo before, but since it's been some considerable time (read : the whole pandemic) since previous posts about this, I won't expect you to go trawling through old posts for details. Oh, you can if you want, but there's no need, I'll just bring you up to speed right now.


Some background : when galaxies get naked

A typical galaxy consists of a disc of stars and some gas all bound together in a dark matter halo. Even outside galaxies space is never truly empty, and especially in dense clusters, the external gas can be significant. It tends to be substantially less dense but also a lot hotter. If a galaxy moves through it fast enough, it can experience a pressure strong enough to dislodge its own gas in a process worryingly known as ram pressure stripping

Sometimes, though rarely, the stripped gas can be so dense that you get a star-forming "wake" behind the galaxy as it moves through the cluster. Normally you can't see this gas using optical observations, but have to use other wavelengths like radio to see the stripped gas trails.

While this might happen to some extent in all galaxy environments, clusters are where it really matters : here the external gas is relatively dense and galaxy motions are (by far) the most rapid. Other processes like gravitational encounters between individual galaxies become much less important compared to the enormously strong ram pressure, which can completely strip a galaxy of all its gas in less than an orbit. 

After that... the galaxy is doomed to a slow, lingering death. With no remaining gas it simply can't form any new stars. Its youngest, hottest, bluest stars soon die off, leaving behind only the smaller, dimmer, red stars. And without the mass of the physically thin but dense gas disc helping to hold them together, the random motions of the surviving stars eventually destroy any hints of structure in its stellar disc. Eventually, it turns from a magnificent blue sparkly spiral into a pathetic red elliptical. Everybody hates ellipticals so it becomes a social pariah and never gets invited to parties anymore. Not even during lockdown. No, not even at Downing Street.

The details of the process are controversial, but the basics are accepted well enough :

  • Ram pressure is strong enough in clusters to cause even massive spirals to rapidly lose all their gas
  • Spiral galaxies in clusters typically have much less gas than those elsewhere
  • Other mechanisms can't seem to explain the gas loss.
Which all leads to the main conclusion that ram pressure plays a dominant role in galaxy evolution in clusters.

Some time ago I was co-author on a paper that attempted to model this process using nice, simple analytical formulae. The gold standard is to do full-on numerical simulations that includes all the complex gas dynamics and stuff, but it seems that the simple formulae are actually plenty good enough to predict the basics. This saves an awful lot of time and, more importantly, effort, because running simulations is annoying. More recently, I was able to show that our model does quite well at matching which specific galaxies are predicted to be currently losing gas and which actually show the long gas streams expected when stripping occurs.

Now I like looking for gas streams very much - looking at data is just inherently a good idea, and it also lets you see what's happening as directly as possible. So you might think that this is just a cunning ploy to let me do more data visualisation.... not so ! For we've also shown that sometimes the gas streams are inherently hard to spot, and the gas disperses quite rapidly. So could there be a different signature of stripping we could look for to test which galaxies have been affected ?

In our latest paper it seems the answer is a tentative but enthusiastic "yes !". We found a radically different test for ram pressure that seems to provide a very pleasing confirmation of the model completely independent of its original formulation.


How to hunt for farting galaxies

The way I mentally group my astronomy knowledge is into three basic categories : atomic hydrogen, stars, and everything else. While stars are what get all the glory, and atomic hydrogen is the largest component of the gas, I'm vaguely aware that there's really quite a lot of stuff contained in the "everything else" category.

You get the idea.


Fortunately, Robert is much more acutely aware of this than I am. For example, we can estimate star formation rates by looking at how much light galaxies emit in different wavelengths. The bluer the light, the more it's dominated by hot, short-lived stars* and so the higher the current rate of star formation must be. We can do this just by looking at broad-band optical filters, much like the RGB components you'd see in an ordinary digital image, or we can user similar filters at shorter wavelengths than visible light (for example, ultra-violet emission is an excellent way to look for some of the hottest stars of all). 

* I am simplifying quite a lot here. Chemical composition also affects colour, as does the total intensity. But these factors can be accounted for.

But we can also do something quite a bit different. Broad filters take in emission from a wide range of wavelengths, but some processes produce photons only over very narrow windows. These "spectral lines" provide another way of testing for the high energies associated with hot young stars. One of these lines, the [CII] ("C two") line, from singly ionised carbon*, has become popular in recent years as another tool in the arsenal of available methods of estimating star formation rate. 

*Any sort of astro-chemistry is something I'd normally steer well clear of, on the grounds that even feckin' hydrogen isn't properly understood, but Robert is a much braver man than I. Personally I think of astro-chemistry some sort of advanced alchemy.

However, [CII] is not emitted directly by stars, but from the interstellar gas. And it turns out that injecting energy into the gas from other sources besides star formation, even mechanical energy, can also trigger [CII] emission... and ram pressure stripping might just be a very good way to do that. 

See, the world of galaxy evolution is anything but woke (though there are some very odd attempts to claim that its offensive language is all due to colonial oppression), and stripping is a violent process*. So slamming the galaxy into the intracluster medium might be indeed be a means for causing it to emit at the [CII] frequency. It's already though that other sorts of gas collision can induce this. Indeed, a paper from as far back as 1999 noted that a spiral galaxy in Virgo had a weird excess of [CII], and this later turned out to be one of the best examples of a galaxy experiencing stripping !

* To the authors suggesting we need to use less violent terms I say OH GOD NO, this is one area in which astronomy is at least still able to give things decent names (unlike new telescopes, which are always called the Very Large Something Or Other). I want my silly jokes about naked bestiality, dammit !

So this bodes well. If we can find galaxies with an excess of [CII], more than predicted from their star formation rates, we can compare this with our model. Since we already predicted which galaxies are currently stripping, we can potentially use this as a completely independent test on whether or not our model is any good.


The results

Spectral line observations are always more challenging than broad-band observations, and the [CII] line is technically difficult. So instead of doing our own observations, we mined the Herschel archive. Of the ~2,000 galaxies in the Virgo Cluster, just 14 had suitable observations. 

So right from the start it was clear that this could only be a limited pilot study. But even given this, initially the results looked confusing at best, and at worst, disappointing. Here's our plot of the excess of [CII] emission as a function of distance from the cluster centre :

Yeah, not exactly the clearest trend in the world... take away the two outliers (VCC 737 and 841) and arguably the "trend" goes away completely. Hmm.

What we might naively expect to see is a clear decline with cluster-centric distance, as the corresponding ram pressure should decrease because the cluster's gas is less dense at greater distances. But it's hard to argue we see anything more than a hint of that, and it's not at all convincing.

You can also see we divided our already tiny sample into two even more miniscule samples : some of our galaxies lie close to the centre of the main cluster (the "northern" sample) while the others are all significantly further away ("southern"). The big blue and purple symbols on the right show the means of the two samples. The southern sample is a control group since ram pressure should be less effective at these distances, while the big green symbol shows another set of galaxies of similar masses but found completely outside the cluster - this forms a second control.

At best, there's a bit of a difference. The [CII] emission is a little bit higher in the northern sample than the others. This is what we'd expect, but it's not exactly an edge-of-your-seat result.

But using cluster-centric distance as a proxy for ram pressure may be too simple. As we'd shown in a previous paper, assuming clusters are nicely symmetrical is about as bad as the proverbial cow :

Or graduate students, according to Fritz Zwicky.

Which is where our earlier, analytical model for ram pressure comes in. Surely what we should do, instead of using that old-fashioned crude approach of cluster-centric distance, is check whether our fancy model tells us if the galaxies are currently stripping or not. With this we should have a much more accurate proxy than simple distance.

Our model calculates two things. First, it estimates how much ram pressure a galaxy should be currently experiencing. This is derived from an earlier model of the gas density within the cluster and assuming the galaxy is moving at about the local escape velocity at its current position. We call this parameter Ploc (pressure at the local point). Secondly, given the mass of gas within the galaxy, we calculate the well-known parameter of deficiency, which just means how much gas it's lost compared to a similar galaxy found in isolation. From this we can compute the parameter Pdef, the pressure needed to reach its current deficiency.

There's a lot of simplifying assumptions in all this, but it gives a nice, simple result : the higher the pressure ratio, the more likely a galaxy is to be currently losing gas. In contrast, galaxies with low pressures may have lost gas in the past but can't be doing so any more. Sounds great ! And remember, this result agreed well when we looked at which galaxies do seem to be losing gas based on their gas streams. But the result when looking at the [CII] excess ?

This is the most boring plot about strippers I've EVER seen.

Yeah, not so much... This was a bit disappointing considering how well the model had worked when looking for gas streams. What was going on ? Does the [CII] just not tell us anything at all about ram pressure but only star formation ? Had we messed up somehow ? Should we go and hang our heads in shame ?

Probably not. Actually, we'd probably over-complicated the situation. The model was constructed in the framework of the usual way of looking for for stripping, by directly searching for lost gas : either just be measuring how much gas a galaxy had, and/or by seeing if it had any detectable gas streams due to stripping. The pressure ratio works well for that scenario because that's the very thing it was based on. But for just injecting energy into the galaxy, which is what the [CII] is sensitive to... that won't work. The pressure ratio is almost irrelevant here : much more important is simply the current pressure. That, not whether a galaxy is losing gas or not, is what dictates the injected energy - which is what might provoke the [CII]*. And when we plot that :

* For example a galaxy which is currently not actually losing gas, i.e. having a low pressure ratio, might still have a high absolute value of ram pressure. The ideal would be to work out how much energy is being injected and how this relates to [CII] emission, but this is a much bigger task. The point is that pressure should be a much better proxy, and doesn't relate linearly to cluster-centric distance.

Bingo ! Now that's a nice clear trend, especially considering the tiny sample size - and the difference between the two sub-samples is stark. Granted there's one weird outlier, which we're unable to explain, but it'd be surprising if there wasn't. If you don't have one weird outlier in observational astronomy, everyone laughs at you or calls you a liar. Or both.

Actually we were a bit surprised by just how clear this trend is. The model is by design simple, and subject to many uncertainties. One of the biggest is that it still does have to consider projected distance (that is, distance on the sky) from the cluster centre when calculating pressure - it can't use true 3D distance, because we just don't know it. This means the true pressure can always be lower, since the galaxy might actually be a bit in front or behind the bulk of the cluster gas. Yet it works even so.

And the trend seems to continue down to very low ram pressures indeed. So ram pressure might be having an effect even in much less dense environments than clusters, like groups. This is perfectly possible, it's just surprising to see such striking evidence of it. Yet as far as we can tell, there is no good reason to expect this trend to be due to anything mundane : there is no selection effect artificially restricting us to galaxies which only appear to follow the trend but actually do so only by chance. 

We also accounted for the fact that some of the [CII] excess will be due to star formation. The thing about ram pressure is that although on sufficiently large and long scales it becomes very simple (all gas gone => no more stars !), on small and short scales it becomes fiendishly complex. As the pressure builds, it can initially compress the gas disc, temporarily triggering an increase in star formation. Then you get all kinds of terribly turbulent structures developing, which look nice but can't be modelled without proper simulations. But we can account for how much the star formation - whatever its cause - increases the [CII] emission, and find that it isn't enough. The simplest explanation is that we're seeing the direct impact of the ram pressure on the gas - we're not seeing galaxies with an excess only because they happened to have high star formation activity.


Conclusion


I wouldn't want to oversell this, because it's still based on a very small sample. But there's an almost startlingly nice trend between our prediction for how much ram pressure a galaxy is experiencing and the intensity of its excess [CII] emission. So our happy-go-lucky, simple model of ram pressure is vindicated for a second time by a totally different method.

It also means that this could be a new way for looking for the signatures of the effects of environment. Even if the ram pressure isn't actually strong enough to cause gas removal, it seems we can see its effects using the [CII] line. And since this appears to happen even at modest pressures, this could apply not just in clusters (where it's a bit of a case of "big bloody deal, we knew ram pressure was happening anyway"), but also in groups and filaments, where the situation is much less clear. That gives us a new way to examine the external influences acting on galaxies.

But again, 14 galaxies ! That's a sample size less than a full-strength rugby team, for crying out loud. So let's not go nuts : it remains a pilot study, a case of, "hey, this seems to work, let's try this some more and see where it goes", not, "everybody just go home now because we're done."

There are two important caveats to end on. First, an earlier study didn't find any correlation between environment and [CII], though this is probably because the authors (a) had less sensitive observations; (b) targeted larger galaxies, which would require more injected energy; (c) didn't consider the pressure parameter. Second, more problematically, getting [CII] observations is difficult. This is something that the SOFIA telescope would be very good for, so let's just hope NASA don't decide to do anything daft like cancelling it.

(Oh, and an apology. It has only just struck me that the title, "Environmental effects in Herschel observations of the ionized carbon content of star forming dwarf galaxies in the Virgo cluster" is probably one of the most boring titles we've ever done. Can't win 'em all.)