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.