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,

Saturday, 10 August 2019

Tracking The Gassy Stripper Parent Of An Orphaned Rhino Through WAVES

There's a question in Cards Against Humanity, "What's that smell ?" to which the answer is always something hugely offensive. And in any large group of people, at some point everyone starts wondering not what the smell is but who it's coming from.

Radio astronomy has a similar problem, which is basically the same except in all the important points, and we aren't content to let such hugely important questions remain unanswered. We WILL find the source of that gas, dammit ! But sometimes, as in the latest paper on which I'm co-author, this can be surprisingly difficult...

Galaxies are not just a collection of stars swirling about in space like gigantic fidget spinners. In fact, the only two similarities they have to fidget spinners are that they're flat and spin by rotating around a central point. But galaxies don't rotate like solid objects, where each part takes the same amount of time to complete one revolution. Instead, stars at different distances take different amounts of time to complete one orbit. By measuring distances and speeds, we can work out how much mass is in the galaxy in order for it to hold itself together :

The answer to this generally turns out to be, "a buttload". The movie on the right shows a galaxy with about the same total mass as all the visible stars, with the one on the left showing one with a bunch of unseen "dark matter". And that's what observations show : they rotate way more quickly than if they were just made of stars (neither looks anything like solid body rotation either).

Of course, galaxies aren't just made of stars, that'd be silly. One of the other key differences between galaxies and fidget spinners is that fidget spinners don't tend to look different if you take a long exposure photograph - pointing your camera at it for a whole month won't make it look any bigger*. With galaxies it's different. The longer the exposure, the more stars you can see. This is difficult, but in general, deeper images make galaxies look a lot more interesting. One of the best known examples is NGC 5907, which has these spectacular double loops, likely produced by an interacting dwarf galaxy.

* That's what she said.

Though the existence of some of the features visible here is now somewhat controversial.
What about gas ? Here too things get more complicated. Gas comes in three different delicious flavours :
  1. Atomic. Just a bunch of atoms of different elements flying about.
  2. Ionised. Gas which has had its electrons stripped off, usually because of hot young stars who are willing to do anything for money which blast high energy photons through the gas.
  3. Molecular. When the atoms are nice and cool, they huddle together for warmth combine to form molecules.
Gas can be made of many different elements. One of the easiest to observe, partly because it's the most common, is atomic hydrogen. Observing this needs a radio telescope. Whereas the wavelength of visible light is measured in billionths of a metre, atomic hydrogen radiates at 21 cm. Catching waves that big takes a suitably gargantuan telescope and a lot of exposure time. Just like with optical images, not only do radio telescopes make maps (and not "listen" to the data cos that'd be stupid), but we also see that galaxies look different the deeper we go.

The nearby galaxy M33. The animation starts with an optical image and then overlays the gas at increasing sensitivity levels. Just like with stars, since what we see depends strongly on our precise observing methods, this makes data visualisation very important in how we interpret the results. That's why we're gonna see lots of different maps of the same object later.

How far does this go ? We don't actually know. Getting the above image took five years with the world's largest radio telescope, and it still doesn't seem that we've found the edge of the gas disc. All the really dense gas is in the middle, where the stars are, but while the density decreases with distance, it never seems to reach a definite edge.

In the middle stage of the animation, you can see there's a loop of gas extending north - evidence that this galaxy has probably interacted with... something... at some point. We don't know for sure what's causing it, though it could well be the Andromeda Galaxy M31. But other such loops are much, much bigger and more impressive. Here's a map of the Virgo cluster - the nearest rich cluster of galaxies to us, with over a thousand crammed into a volume not much bigger than our own Local Group :

Blue points show spiral and irregular galaxies while red ones show ellipticals (the faint green ones show especially faint galaxies). Arrows show known hydrogen streams. Considering how many galaxies are present, the number of streams isn't that large, but that's a topic of another paper. Just north of the grey shaded area you can see a feature labelled as "Kent 2009", which is one of the most unusual features known.

This giant gas cloud was discovered by a certain Brian Kent using the ALFALFA survey at Arecibo. He found that there's a collection of possibly linked clouds, spanning about 150 kpc (just shy of half a million light years) with around 500 million times the mass of the Sun in gas, with no obvious connection to any galaxies. The Kent complex* is big, but isn't the biggest or most massive known gas cloud - but that it's so completely detached is very unusual. The other gas clouds have their own mysteries, but at least we can usually identify the culprit. But for the Kent complex, it's very much a case of, "who let that one out ?"

* We originally called it this in the paper, but Brian doesn't want it named after him so we changed it. 

What we need is more sensitive observations. That's where the Widefield Arecibo Virgo Extragalactic Survey comes in, which is about four times deeper than ALFALFA. Just for the record, I suggested a bunch of other names but they were all rejected :
  • BESTEVER : Big Exciting Survey To Explore the Virgo Extragalactic Region
  • WIVES : Wide-field Interesting Virgo Extragalactic Search
  • ADHESIVE : Arecibo Deep Hydrogen Extragalactic Survey In Virgo Environment
  • VADER : Virgo Arecibo Deep Extragalactic Research
  • THIEVES : The HI Extragalactic Virgo Exploration Survey
  • WHATEVER (my favourite) : Wide-field Hydogen Arecibo Telescope Exploration of the Virgo Extragalactic Region
  • RIVETED : Region In Virgo Explored To Extreme Depth
  • DISHWATER : Deeply Interesting Survey of Hydrogen in a Wide-field Arecbio Telescope Exploratory Region
  • MOVES : Mother Of all Virgo Extragalactic Surveys
Oh well. Anyway, more sensitivity = more chance of finding fainter gas = more chance of finding the culprit. This works equally well if you go for Robert Minchin's analogy of gas streams as being the bloody entrails of galaxy interactions and want to hunt down the murderer, but farts are less disgusting.

It didn't work though. We did find more gas - quite a lot more, actually, bringing the total to about 1.5 billion solar masses. And we confirmed that the clouds are indeed all embedded in a common envelope of gas - they're not just a bunch of clouds that happen to be having a party. Rather, what Kent saw were the brightest parts of a single structure. Here's a comparison :

Left : Kent's original map. Right : our shiny new WAVES map.

Actually, in our data set the complex looks for all the world like a collection of galaxies, with this fainter emission being evidence of an interaction. But that's highly unlikely : there's no evidence of any optical counterpart at all, and Kent also did higher resolution observations which showed that no parts of the structure are rotating. So it's not a complex of interacting dark galaxies. At this point I'd usually show a movie, but nowadays I can go one better. Here's an interactive 3D model !

You can read more about these models here. I wanted to show more intensity levels than the three here, but isosurfaces in Blender involve a lot of manual work and I had problems with layers intersecting each other and it looked horrible.

Can't run it ? No problem, here's a really pretty renzogram from the paper. Each colour shows the hydrogen detected at a slightly different frequency, which is equivalent to velocity - and not the same as distance. So don't go thinking the 3D model shows the true shape of the cloud along the third axis. It gives very interesting information about how the cloud is moving, but not about its 3D shape.

So what is this bloody entrail/giant space fart ? Kent suggested it could have been stripped out of some of the nearest galaxies in this region. We didn't find any evidence of a direct connection, but it still seems that this is the most likely explanation. But it's not at all easy to identify who's responsible. Here's a map of the surrounding region :
Each galaxy has been coloured according to its average velocity. Galaxies we didn't detect are shown in ellipses. The thin grey lines show the edge of our survey region.

And note that in this particular rendering, it looks quite a lot like a space rhino :

Given the rainbow colour scheme in the renzogram, this is clearly a giant gassy space unicorn. Real unicorns have curves and are gassy. #Diversity
All of the other known streams are connected to their parent galaxies in a head-tail structure, with the galaxy found at one end. So the galaxy supplying the gas to the space rhino ought to be found somewhere near either end of the stream and roughly aligned with its longest axis. Kent's suggested parents don't do that, they're practically at right-angles to the long axis, and that's weird. It's hard to imagine the kind of orbit that would produce a feature like this. One of the proposed galaxies even has a tail pointing in exactly the opposite direction, which basically rules it out completely.

And there's so much gas in the stream that raises a further problem. Just as galaxies don't consist of just stars, neither do clusters consist only of galaxies. They're also filled with their own gas : much hotter and thinner than that in galaxies, but with an enormous total mass. Galaxies moving through this material at a high enough speed can have their own gas pushed out (ram pressure stripping), where it can slowly disperse and evaporate. So for most galaxies with streams, only a few percent of their gas mass is found in their streams : once their gas leaves the safety of their stellar disc, it's vulnerable to being torn apart and heated to the point of ionisation.

But there's so much mass in the Kent complex that this doesn't seem to have happened. Nearly all of the gas that the proposed donor galaxies would have had is still in the stream, and doesn't seem to have evaporated at all. Oddly, it's the lack of missing gas that makes the Kent complex even more unusual - it's not at all obvious how the gas has survived. Alternatively, some fraction of the gas could have evaporated, but in order to explain the enormous amount of gas that survives, that would have made the parent galaxies exceptionally gassy before stripping. And no-one, no-one at all, ever wants to be gassy before stripping.

Actually, there is one galaxy which almost fits the bill as a parent : NGC 4522. This is in the right place and even has a tail pointing towards the Kent complex. But if this is the parent, then it's an errant one that's trying to escape real fast : it's moving at 1,800 km/s relative to the complex. And why it's disconnected is unclear. Maybe it just wasn't ready to deal with parenthood, but 1,800 km/s is so fast it means they're probably not associated at all.  And there are plenty of other galaxies with tails in this region, so the alignment could just be a coincidence.


At this point we're fast running out of options. We tried to track down the parent, but none of them are claiming responsibility. Some are in the wrong place, some are too small, some too far away, some moving too quickly, some are moving in the wrong direction. None of them are at all satisfactory.

We do at least have a least-worst option though : NGC 4445, one of Kent's original suggestions. It's in the wrong place and would have had an awful lot of gas (or there would have to be unknown processes at work preventing the gas from evaporating), so it's not a great candidate. Not at all. In fact it's just plain lousy, but all the others are even worse.

A more radical suggestion is that the galaxy stripped in an unconventional way, losing its gas in a rapid event which then dispersed, so the long axis of the stream doesn't point to its parent. That's something we're still trying to model. It would open up more possibilities but it doesn't seem terribly likely either.

About the only good news is that the complicated velocity structure of the complex is something that we can explain. In simulations, those sorts of features are pretty common : once gas is removed, other galaxies can harass it into all kinds of complicated shapes. That doesn't help us with any of the other problems, but at least it's not a complete mystery.

So our best guess for this orphaned, harassed, gassy space rhino is that its errant parent was a stripper who refuses to claim responsibility for their offspring. Astronomy : it's every bit as entertaining as the Jerry Springer show.

Yeah, but are you a stripper who's failing to claim parental responsibility for their gassy space unicorn ? I think not.

Thursday, 8 August 2019

Big Astronomy In Little America

Look, another travel post ! Not a long trip to foreign parts, but a nice walk in the Czech Republic.

One of the most popular Czech tourist destinations is Karlstejn Castle. It's very nice, look :

Pretty sure I've already described visiting Karlstejn so no need to do that again. It was nice to see it under a more interesting sky this time, and it seemed like a good place to take a visiting seminar speaker on the weekend.

I should say that this year June was ferociously hot (up to 36 C) while July and August have been much cooler (generally not above 27 C, and even dipping down to 11 C on some nights). That's completely the wrong way round from normal, not that I'm complaining. The day before had been one of the hotter days, so the overnight drop had us convinced we'd picked a better day for it.

The guided tour of the castle takes a bit less than an hour, so after that we planned to visit a place called Velk√° Amerika (Big America, even though it's quite a lot smaller than the real America, but there's also a Little America and I think a Little Mexico). Getting to this abandoned quarry is fair walk from Karlstejn, though there are only one or two steep parts. The weather, however, got steadily angrier that a bunch of astronomers dared encroach the fields of wheat, even though none of us felt inclined to run through them in case that upset Theresa May.

Such skies are typically found on the covers of metal albums so we decided to form a band. Our first album we be called Core Collapse Supernova. It will feature such tracks as :

  • Strange matter
  • The sun goes down before the sun goes down
  • Hot mode accretion
  • Cooling flows
  • Dark energy
  • Superluminal motion
  • Violent relaxation
  • It's raining on my black hole
And so on. We made really stupid band photos of us posing dramatically, but unfortunately I don't have any of those. Ah well.

The weather continued to ramp up the drama :

But by the time we got to the quarries the sky looked less like the fury of the gods was about to wax forth. We got some very nice views, for a few minutes.

There are really serious levels of edge involved here. One false move and you won't be making a second false move. It's a sheer vertical drop of 100 metres, no handrail, no ledges, just down.

Fortunately this trip didn't become the first time we'd lost a visiting speaker. But then the sky decided that we'd seen enough and it started to rain. The path along the edge is not especially wide, so we bid a hasty retreat. This was a wise move. For a little while it was bearable underneath a small umbrella, but then the gods decided that everyone needed a good hard smiting. Thunder boomed. Lightning flashed. And the sky emptied.

We carried on as far as we could, but then we reached a steep, muddy descent. It didn't seem like a good idea to try and go down while the sky was exploding, so we didn't. There was nowhere to shelter, so we didn't do that either. We simply stood there, getting wet. Pretty soon we were all soaked through and somewhat chilly. Eventually the rain decreased to the point where standing still in the cold heavy rain was marginally less appealing than walking down a steep muddy slope in the rain, so we did that. Very slowly. Everyone survived, albeit acquiring various amounts of mud in the process.

Eventually the rain stopped completely and the Sun came out. The water now had nowhere else to go, so it went back the way it came : upwards.

After that we trudged on somewhat grimly back to a three-headed dragon pub, where , still soaked, we had a most welcome and nourishing meal of extremely delicious pancakes. The sunshine continued apologetically.

Not that it felt like a very sincere apology, more the sarcastic mocking kind that says, "hah, I'm too weak to dry you off, so I'm going to taunt you instead". The chilly, damp train ride back to Prague was followed not by watching the Zizkov T.V. tower illuminated to commemorate the Moon landing anniversary (I'd seen it before anyway) but with a nice hot bath. And that I can state with certainty was the better choice.

Thursday, 18 July 2019

The tortuous travel terrors of Tenerife touring

Yay, travel post !

The recent set of horrendous delays have made me much more trepidatious about travelling. Sure, being stuck in an airport isn't as bad as a brick to the head, but it's still not very nice. But I have a bunch of travel money now, so I'm compelled to go places... although perhaps in future I can just delegate my postdoc to do it for me. We'll see.

Being somewhat paranoid since easyJet* have this demented policy that you can only have one piece of hand luggage, even if the other piece can go under the seat in front of you**, I packed lightly enough that I could get my laptop in my suitcase if need be. On this occasion I needn't have worried - it turned out my ticket allowed no less than three pieces of hand luggage anyway, and all the flights went smoothly. Three hours to Madrid, four hour layover***, three hours to Tenerife for the conference. Boring, but not awful.

* I flew Air Europa and Czech airlines this time.
** They don't usually enforce this, but you never know. That it's even a policy at all is ridiculous.
*** You need at least two hours in Madrid to be sure of getting a connecting flight, because it's the size of a small city and - although otherwise nice - it's very badly signposted.

There seemed to be an awful lot of lakes and rivers in Spain with highly fractal structures.

My only point of confusion came when I tried to walk from the airport to my hotel, which Google Maps says is a 40 minute walk - just what I need after being stuck in a tiny space for three hours. But it isn't a 40 minute walk. It's a 40 minute death-defying game of chicken between you and the cars on the busy highway. The lack of pavement was fine on the quiet backroads, but even after following the directions of a local, I was soon forced to admit that the only way forwards would be essentially to walk along the hard shoulder of a motorway, and this didn't appeal. So I gave up and went back and got the bus. The only difficulty there was the rather surly-looking (and sounding) bus driver, who tried very badly to explain that I needed to put my suitcase in the storage bin, rather than actually opening it himself unlike every single other bus driver I've ever encountered.

If you're thinking of visiting Tenerife, you definitely should. But be advised that La Laguna is not a very touristy place : it's nice, but it simply doesn't factor English-speaking tourists into its equations.

Fortunately, having travel money to burn, my hotel was a nice one. Things are weird like that when someone else is paying. Sometimes you get places so bad that the only good thing about them is that they don't catch fire. And at other times you get opulent luxury.

This place wasn't quite opulent (at least my room wasn't, though the lobby was), but it was very good. The restaurant, however, turned out to be probably the best I've ever had. For 12, every day I had a massive buffet breakfast and ate too much ALL the cake - and it was all delicious. The chocolate croissants were so chocolately that I'm actually glad you can't get ones that good anywhere else, otherwise I'd be stuffing my fat face with them right now. And for 16 I could get a three course meal which was equally monumental.

Anyway, I didn't have a lot of spare time on this trip. I hadn't planned anything, so for my only free day I wandered around randomly - which I maintain is an absolutely essential part of any tourism. And like I said : La Laguna is nice, but there's nothing much to actually do. One day was nice but quite enough.

Tenerife has a crazy variety of trees, most of which look like giant pineapples.

Inevitably it reminded me of Puerto Rico, though the temperature was only about 20 C and the drivers actually know how to operate a car.

Walking around La Laguna's narrow streets, I caught frustrating glimpses of a more impressive landscape beyond. Too often the buildings seem specifically chosen to obscure a perfect decent view instead of revealing it. Having not much luck walking uphill, I decided to go down. This gave a deceptively good view of the sea, which looked close but was actually many miles away. Fortunately, there was one nice loop of road which does nothing at all except provide a rather nice view. Quite literally : it doesn't take the traffic anywhere it couldn't go faster by another route.

Later on I decided to try a different direction. After a couple of failed attempts to get up the hill, I found a route which took me somewhere nice. There was absolutely nothing there, but it gave me the proper vantage point I'd been seeking.

The science bit

Before the conference could begin I had to do remote observing, starting at 3:45 am and running until 5 am. Oh, yay. The first night this simply didn't work at all, and all I got for my trouble was a loss of phone credit as I tried multiple times to phone for the password. I tried the hotel phone and got a confusing American voice telling me that "all circuits are busy". After about half an hour of this, having had little enough proper sleep, I gave up and caught another two hours before I had to head out to the conference.

I found out the next night that it wasn't a problem on my end at all, but with Arecibo's phone lines : there was no way I'd have been able to connect. The next night I had no connection problems, and was rewarded with the usual CIMA screens :

It isn't actually as bad as it looks, but at 4 am with little sleep two nights in a row it's quite bad enough. There was a blissful 30 seconds when everything seemed to be working perfectly, and then warning messages started which got worse and worse until the scan failed in a way I've never seen before. The rest of the session was spent fruitlessly trying to find the problem and failing, although the operator seemed to nail it down eventually.

The third night things got off to a slow start, and I strongly suspect the first scan is useless, but the second one seemed okay. So for all this effort I managed to get one scan of... a void. Yay. Add to this the intense conference schedule (9am - 7pm, longer on some days) and you can imagine that I was quite tired. I say that this is what real science is : working 10 hour days and getting up in the middle of the night to get a scan of a field you're almost certain doesn't contain anything interesting and you don't get to find out the result for several years. There are some compensations, mind you.

What of the conference itself ? It was first rate. I've written a summary of the highlights so I'll largely skip over this here. In brief, there were quite a lot of, "here are the problems we face and the ways we're planning to tackle them" talks, perhaps a bit excessively so, but there were also plenty of really interesting science talks too. The major themes were that galaxies look remarkably different at low surface brightness levels, that we're going to face major data challenges in the years ahead, that we don't fully understand ultra-diffuse galaxies yet but we're definitely making progress, and that the future's bright dim. Way too many people made that joke.

Back to being a tourist

Okay, science over. But the conference also included a dinner in a five star hotel, situated near a lovely park albeit with a weird fountain...

I've seen many a weird statue, but this is the first that goes for figures with less than ideal proportions. Good for you, Tenerife.

How does one tell the difference between a four and five star hotel ? Simple - the five star one has an outdoor turtle enclosure.

Dinners are nice, but uninteresting. More dramatic was the excursion to Teide Observatory. We saw a solar telescope, a pyramidal sundial, a 1.5m optical telescope and a few other instruments scattered around the site like mushrooms. But mainly we saw spectacular views of the volcano, towering above the sea of clouds a thousand metres below.

It looks like what would happen if the Teletubbies met Hobbits and went off together to live in space.

The solar telescope has an enormous vacuum tower because the light it collects is so powerful it would otherwise generate its own seeing.

I've got no clue what this is. I just thought it made for a nice minimalist shot.

All this was finished off with tea and biscuits at 2400m altitude.

A high-tech facility with a volcano ? I suppose it's like a sober version of James Bond.

The final day consisted of an optional trip to the observatory on La Palma, which I'm very glad I did. Although the Teide volcano is much higher at 3700m, the summit of La Palma overlooks a huge volcanic caldera. It takes a while to get there though. First there's an 8am flight which takes slightly longer to taxi to the runway than it does to hop between islands (it actually flies at a slightly lower altitude than the summit of Teide)....

Then there's an hour's drive in a bus up a very windy road (and I lived in Puerto Rico so I know what I'm talking about when it comes to windy roads) but with some outstanding views.

Finally we got to the observatory complex, which had laid out a much-needed breakfast. Tea and croissants and cake aplenty. The rest of the day was spent on a tour of the telescopes. Our guide was Sheila, an extremely mumsy sort who I had the distinct impression could easily be site director if she wanted. We started with the giant MAGIC telescopes. These are enormous, 17 and 23 m diameter optical telescopes designed to detect Cerenkov radiation from gamma-ray bursters and other exotic, high-energy sources.

It's fun to watch the reflections change as you walk around, especially on the one with all the mirrors slightly out of alignment.

Of course the nice thing about these instruments is that anyone using them gets to call themselves an expert in MAGIC.

The thing about Cerenkov radiation is that it's produced by particles within our own atmosphere - they don't receive photons direct from the source as other telescopes do. So their resolution is absolute crap. The game here is sheer light-gathering sensitivity, which means the segmented mirrors don't have to be precisely aligned since they don't try to produce a focused image. Which is why they're so much bigger than other optical telescopes. Mind you, they're not so unfocused that they couldn't accidentally be turned into a genuine, no-fooling-around death ray, which is why they're pointed well away from the Sun.

Next we went to the world's actual largest optical telescope, the Gran Telescope Canarias. It's run at a mere $6 million per year - compared to the slightly smaller Keck facility which consumes $17 million, this is not so much impressive as it is worrying. That's the sort of level where it's astonishing they're able to keep the lights on, and sadly explains why this place doesn't get much publicity and hasn't made any major breakthroughs as yet. It's not that it doesn't have the technical capabilities to do so, it just doesn't have the funding. That doesn't stop it being a hugely impressive piece of engineering though.

The dome of the GTC rotates with less noise than a conventional car. The telescope itself can be moved around by hand and is absolutely silent.

Our final telescope was the venerable William Hershel Telescope. Though a 4 m diameter is now small compared to 10 m giants like the GTC, it's a reliable old workhorse laden with all kinds of different instruments. And it still manages to impress through sheer gargantuan size - older telescopes tending to have much larger domes than they really needed.

Saving the best till last, we finished with a walk around the summit. Although 2400 m is more than a thousand metres below the highest altitude I've reached, it was still tough going walking around. We had only a little time, but it was enough to see the highlights. Not that the photographs are really adequate, mind you. You get a much stronger sense of oh my goodness that's a long way down isn't it? up there.

And then I went home, quite failing to have any plane or luggage problems at all, and collapsed in a big heap.