Huge dwarfs, not crouching giants

Back in October I summarised the discoveries of large numbers of extremely faint galaxies in the Coma and Virgo clusters. Since then there's been a steady stream of papers on these ultra-diffuse galaxies, at the rate of about one a week. They now seem to be a common feature of many galaxy clusters (e.g. this one).

Although low surface brightness galaxies have been known for many years, these galaxies are much larger - as extended as the Milky Way but a thousand times fainter. The important question is how massive they are. They could potentially be either huge dwarfs (very extended but not very massive) or so-called "crouching giants") (as massive as giant galaxies but with almost all of their mass in dark matter rather than stars). If they were crouching giants, they could be a huge challenge* to cosmological models, which don't predict anything like these objects in such numbers.

* As in, "holy crap what the hell happened ?"

A few very recent developments indicate that these objects are likely to be huge dwarfs. The difficulty has been that to measure the total mass you need to know how fast the stars are moving (to calculate how much mass you need to hold them together). For objects this faint, this is extremely difficult, especially since the little blighters don't seem to have any gas (which is much easier to use to measure motions).

A paper in February of this year used a neat trick to make things easier, measuring the speeds of the globular star clusters of one of these galaxies rather than individual stars. Still difficult, but easier. They found that although the galaxy is extremely dominated by dark matter (around 3,000x as much mass in dark matter than stars - for normal galaxies it's more like 10x), it's just a dwarf galaxy after all.

They also found that they could get a pretty accurate estimate of the total mass from the globular cluster mass - confirming that a relation known to work for ordinary galaxies works even on these very faint objects. Which means we can completely avoid measuring the motions of the stars altogether and essentially just measure the brightness of the globular clusters. Huzzah !

A paper published today  uses the globular cluster trick to confirm that another similar object is also a huge dwarf, while a paper from last month shows that these objects are broadly compatible with mainstream theoretical models of galaxy formation. So although we don't have too much data to work with, it seems likely that most of these objects are faint, low mass, but very extended.

Not that this means everything is hunky-dory. Far from it. Although the vast majority of these things don't have any measurable gas content, a very few are extremely gas rich. Why some should have gas (yet apparently are not currently forming any stars) while others have managed to lose their gas completely is a mystery. And still, as a hugely under-cited paper makes clear, the number of these objects is far lower than cosmological models predict. They may not be quite as dramatic as blowing all our ideas out of the water, but they're still bloody interesting little buggers even so.

Boaty McBoatFace instantly disproves all charges of elitism

I'm sort of tempted to start an e-petition to compel them to actually use the name. Yes, the government couldn't directly force them to choose the name, but I'll bet the science minister has some influential clout.

Why I think they should actually use the name instead of dismissing it as the internet having a laugh :
- By having the audacity to use such a silly name for such a serious project, it will project supreme self confidence to the world.
- It will instantly appeal to a huge number of young taxpayers who would otherwise not have cared.
- It's a conversation starter. The name buys you a massive outreach opportunity that a polar explorer or a naturalist just doesn't.
- It will instantly and forever shatter the image of the elitist, out of touch, boring scientist. I can't emphasise enough how important that is. Also this : http://www.forbes.com/sites/startswithabang/2016/04/16/ask-ethan-why-dont-you-look-like-a-scientist/#61aedb12272c
http://www.bbc.co.uk/news/uk-england-36064659

Paper submitted !

Just submitted my sixth paper as first author. I suppose it won't hurt to show the title and  abstract :

Attack of the Flying Snakes : Formation of Isolated HI Clouds By Fragmentation of Long Streams

The existence of long (> 100 kpc) HI streams and free-floating HI clouds (lacking a clear association with a parent galaxy) is well-known. While the formation of the streams has been investigated extensively, and the isolated clouds are often purported to be interaction debris, little research has been done on the formation of optically dark HI clouds that are not part of a larger stream. One possibility is that such features result from the fragmentation of their more extended parent streams, while another possibility idea is that they are primordial, optically dark galaxies. We test the validity of the fragmentation scenario (via harassment) using numerical simulations. To ensure that our results correspond to observations, we present catalogues (from literature searches) of both the known long HI streams (42 objects) and free-floating HI clouds suggested as dark galaxy candidates (51 objects). In particular, we investigate whether it is possible to form compact (< 20 kpc) features with high velocity widths (> 100 km/s), similar to observed clouds which are otherwise particularly intriguing dark galaxy candidates. We find that producing such features is possible but extremely unlikely, occurring no more than about 0.2% of the time in our simulations. In contrast, we find that genuine dark galaxies could be extremely stable to harassment and remain detectable even after 5 Gyr in the cluster environment. We also discuss the possibility that such objects could be the progenitors of recently discovered ultra diffuse galaxies.

Now I have to figure out what the heck to do next...

For the record...

I'm just going to go on the record and state that I think planet "9" is a silly thing, and a year or two from now no-one will care about it any more.

A new explanation for optically dark gas clouds

I am not at all biased in any way that this paper discusses some of the optically dark hydrogen clouds found as part of my thesis project. I care not a jot than an Einstein fellow and a Harvard professor have decided that these objects are worthy of investigation. I am definitely being totally objective in choosing to report this paper.

What the authors do here is to look at the hot intracluster medium in the Virgo cluster (detected through X-rays) and the optically dark hydrogen clouds detected through radio surveys. They analytically predict the sizes of the hydrogen clouds if they were in pressure equilibrium with the hot gas (that is, not expanding or contracting). Why should they be in equilibrium ? Well, if they weren't, they'd either expand and become undetectable, or collapse and form stars. Both process would be quick, so it's not likely that we're detecting them in this narrow interval. If they're stable, they'd be long-lived and therefore it's much more likely that we'd detect them.

To be in equilibrium, they find that the true sizes of the clouds would be compatible with the observations. Currently we only know the maximum sizes of the clouds because Arecibo's resolution isn't good enough to determine their true extent. They also find that the clouds would be just below the threshold at which they should start forming stars. There's a bias towards detecting clouds near this threshold because if they had any less hydrogen they'd be undetectable - so there could be more small clouds we haven't detected.

What they don't discuss, which you'll have seen recently if you follow my Astronomy Graphics collection [now defunct due to the end of Google Plus], is the origin of the high velocity width of the clouds. As I've shown through extensive simulations over the last year, this is damn difficult to explain through tidal encounters, but easy if the clouds have their own dark matter halos (exhaustive 27-page paper to be submitted next week). I'm not sure what the pressure equilibrium condition means for the dark galaxy scenario - probably that turbulence in the hot gas is another interpretation of their high velocity widths. On the other hand I don't see that this necessarily precludes the possibility of a dark matter halo.

In any case, they emphasise the need for follow-up observations, which ought to make things easier come the next observing proposal deadline.
http://arxiv.org/abs/1604.01767

Testing PyQTGraph

Having nothing much else to do while I wait for certain lazy co-authors such as Robert Minchin to respond, my afternoon has been spent investigating PyQtGraph.

Took < 5 minutes to install and about 15 minutes to hack the example code to load my own data set. Doesn't look so impressive in the screenshot I guess, but it's a realtime volumetric render, so I can move the display around easily. The really impressive thing is that it took all of 10 seconds to load the data - FRELLED would take more like 2 minutes for a cube this size. I suspect it's using a similar technique to FRELLED - mapping the data on to planes - but with a vastly more optimized engine.

Of course, I have no clue how to change the colour scheme or do anything useful with this, because the code is uncommented.  But this warrants further investigation.

Maxing it out

Simulation visualisation I did over the weekend for my friend Rory Smith. He has some huge super-fancy simulation showing what happens to a dwarf galaxy as it falls into a cluster. It's a very complicated data set so this one's shown in stages.

First we see the stars as simple points. You can see they're in a sort of grid distribution, but this is just because their positions weren't output to high enough precision. They're rendered very crudely here because Blender has problems combining halo materials with transparent planes. I guess I could probably do something with z-buffers and composition, but time's a-wastin'.

Second we overlay the dense gas in the disc. There's a strong warp in the centre, not sure what's going on there. The peak density is more than a million times the lowest density, and the lowest-density material is found inside the cluster rather than originally in the disc. So it's useful to start by showing only the gas that originated in the disc, otherwise it gets very confusing. Then we see the velocity vectors in the gas, so you can see how it's rotating and being disturbed as it enters the hot gas in the cluster.

Then we overlay the thin gas from the cluster itself. You can see the bow-shock where the gas piles up against the galaxy, and a lot of very complicated, turbulent structures in the stripped wake. Finally we overlay the velocity vectors of this thin gas so you can see just how complicated these structures are (not sure how good the compression is on Google video).

This isn't totally maxing out FRELLED's capabilities, we could also colour the gas by temperature instead of density, do a larger cube or a time series etc. But it's quite a nice way to demonstrate many of the major features.

Galaxies, galaxies, everywhere...

This paper claims there could be a huge population of as-yet undiscovered very faint galaxies. The idea is that they have been previously missed in existing surveys because of problems with identification techniques and survey sensitivity limits.

I know (to varying degrees) all three of the authors so I was privy to an earlier draft of this last year. I already sent the first author some detailed comments many months ago, but the only response I had was, "thanks for your thoughtful contribution". As far as I can tell, my comments haven't been included in this updated version. I'm not sure whether it's worth the effort of sending a second email or not.

There are two serious problems here I want to point out :
1) The idea that neutral hydrogen (HI) surveys have missed a huge number of dark galaxies because of poor identification procedures
This can be falsified using the author's own numbers. Firstly, they claim that, for instance, the ALFALFA HI survey identify optical galaxies as much as 2 arcminutes away from the coordinates from the HI measurements. While true, the median offset is just 23 arcseconds ! So most of the time the identifications are reliable.
Secondly, the authors state the minimum physical separation between the HI and optical coordinates should be < 13 kpc for a secure identification. I looked at the numbers on this, and for the ALFALFA survey within 50 Mpc of the Milky Way just 0.6% of their detections have larger separations. So it cannot possibly have missed a large number of dark galaxies in the nearby universe.

2) Optical surveys have missed a huge population of very faint galaxies
They even give reasons why CCDs can't detect them. It's a little agonising to read because a year or so ago this might have been interesting. The trouble is that these ultra-diffuse galaxies are a hot topic at the moment with papers coming out regularly. We know they exist, and in large numbers.

What's particularly baffling is that the first author was the PhD supervisor of my PhD supervisor (thus making him my grand-supervisor), who's done a lot of work on this himself. They work in the same building. Yet apparently the one is not aware of the other. Oh deary deary me...

So, large optically faint galaxies do exist. They are definitely a thing, with certainty. But large gas-rich optically dark galaxies do not exist, with an extremely high degree of confidence. However, smaller gas-rich dark galaxies may well exist. But that's another story, aka a 26 page paper with > 200 simulations, currently in preparation. :)
http://arxiv.org/abs/1603.02590

Do you feel lucky ?

"As people dredge the unknown, they are engaging in a highly creative act. What an inventor “finds” is always an expression of him- or herself. Martin Chalfie, who won a Nobel Prize for his work connected with green fluorescent protein — the stuff that makes jellyfish glow green — told me that he and several other Nobel Prize winners benefited from a chain of accidents and chance encounters on the way to their revelations. Some scientists even embrace a kind of “free jazz” method, he said, improvising as they go along: “I’ve heard of people getting good results after accidentally dropping their experimental preparations on the floor, picking them up, and working on them nonetheless,” he added."

Observational science is rather different from the hypothesis->experiement->verify->refine/disprove cycle taught in schools. It makes sense to have some idea you want the data to test, but planning anything that's more than vaguely defined is (as a rule of thumb) foolish. Too often the data will just show something you didn't expect at all. Better to approach the data with as few preconceptions as possible and try and determinine what it's telling you without bias. You could try speculating about as many different scenarios as possible but it's usually a waste of effort that ends up being castles in the air. So in that sense, observational astronomy is a great example of an inherently serendipitous process.

Of course, there are exceptions. If you're building a new instrument you need to have some idea as to what it should detect - you can't go spending millions of currency units on some new-fangled telescope unless you're pretty sure it's going to detect something. But more often than not, once you've established that you'll detect something, you let the data tell you about the universe, rather than trying to test a specific hypothesis. Hypothesis testing is useful and has its place, but it is not the be-all and end-all of science.

Which is a very roundabout way of saying, "I don't like writing telescope proposals, they're silly".

EDIT : I was also going to add that with regards to fostering an environment that promotes serendipitous discoveries, I've already written about that here : http://astrorhysy.blogspot.cz/2015/11/when-worlds-collide-science-in-society.html. Relevant quote :
These similarities mean that sometimes the process of doing both science and the arts can be very similar. Both require large amounts of time to do nothing but thinking (and in the case of science at least, an awful lot of background reading). Inspiration can't be forced - you cannot make people have new ideas. You can, however, encourage them. Science and art are both sometimes highly elaborate forms of play, to explore the question, "what if I did it this way...", or better yet, "what does this button do ?" Such thinking intrinsically demands a liberal, reasonably informal atmosphere. Insisting that people are at their desks during some particular set of hours and only talk to each other during scheduled meetings makes absolutely no damn sense whatsoever.

Except that I have a nice personal counter-example. I recently came up with a way to view volumetric data in spherical polar coordinates directly in Blender with no need for Cartesian gridding... during a long and extremely boring meeting to which I was not paying any attention. So perhaps seemingly pointless meetings have their uses after all.

http://www.nytimes.com/2016/01/03/opinion/how-to-cultivate-the-art-of-serendipity.html?_r=0

The ALFALFA Sky II : The Thrilling Sequel

Got my colour issues all sorted. For those who missed the earlier posts, this shows the visible light component of 22,935 galaxies from the ALFALFA hydrogen survey - using the actual image of each galaxy, from the Sloan Digital Sky Survey. Galaxy sizes have been exaggerated by a factor of forty (not five as I earlier reported). More detail in the link below... it's a post from 2013, but I've updated it to include the new data/techniques. The previous version had a poxy 11,710 galaxies, so this one is like, waaay better.

Go straight to the full HD video here, partly in 3D if you've got any red-blue glasses hanging about :