The Ones That Got Away

Since I follow anything relating to dark extragalactic hydrogen clouds with the obsession of a rabid dog, I'm amazed that I missed these. But I did.

This paper is about optical follow-up observations of two extremely massive hydrogen clouds near the obscure galaxy IC 5270. These were previously reported back in 2015. Most such clouds are thought to be debris, removed from parent galaxies either by tidal encounters or other gas stripping processes. There are a handful of dramatic, very extended hydrogen streams known, and quite a few more smaller, discrete clouds, which tend to be low-mass.

Not so these two stonkin' great clouds, both of which have about a billion times the mass of the Sun in hydrogen (a few times less than that of a giant spiral like the Milky Way). They're very close to IC 5270 itself, no more than a few times its optical diameter, which has about 8 billion solar masses of hydrogen. One other galaxy in this group shows a disturbed hydrogen disc, but IC 5270 itself does not. And while it's possible to tidally strip the outermost, thinnest parts of a gas disc without doing too much damage to the rest of the galaxy, it's very difficult to imagine what process could remove so much gas while leaving the galaxy itself unscathed. In other environments, ram pressure stripping (the effect of a galaxy moving through a hot, low density, external gas belonging to the group itself) might be viable, but this group is too small to likely possess much of this - and anyway the motion of the galaxy is just too slow for it have any effect.

Similar amounts of more obviously stripped material are known in other cases, but the key difference is the size. These two clouds are both relatively compact - at about 20 kpc in extent, they're not so different from an a typical spiral galaxy. Stripped material of this mass tends to be very much larger. This means the density of the clouds appears to be comparable to that of standard spiral galaxies, well above the threshold where star formation is normally occuring. Yet they're almost entirely optical dark. The new observations presented in this paper identify some truly pathetic optical counterparts, but these don't even well-match the positions of the centres of the clouds (both look to be in parts of the same cloud - the other remains entirely dark). They may well be associated, but they still challenge the expected star formation efficiency. The authors speculate that the clouds might be more extended along the line of sight, giving them a greater volume and so making the gas less dense than it appears.

So what the hell are these things ? It's very difficult to say. Based on earlier data, there could be a significant amount of extra hydrogen over a more extended area. This is a great weakness of most new major radio telescopes : they're interferometers, which are hundreds of times less sensitive to extended structures - they're limited to the densest gas. This is still very interesting since in these cases the clouds are so dense that they ought to be forming stars, but it means we might be seeing only a small part of the system.

Frustratingly, there's no measurement of the velocity width of the clouds. And that's crucial, because we've shown with extensive simulations that while it's relatively easy to produce a long stream with a high velocity gradient, it's really difficult to produce isolated clouds exactly(!) as small as these with high gradients. They say that one of them has a "slight" gradient and the other is "homogeneous", but they don't quantify this; figures in the earlier paper lack labels for the velocity colour bar. Which is mmwwwwwaaaaargh because this is something for which numbers really matter - double the gradient and it gets vastly more difficult to produce, at least according to our models.

Furthermore, the clouds are both on the same side of the galaxy. Most tidal encounters produce a double tail (just as the Moon produces high tides on opposite sides of the Earth). High velocity encounters can produce one-sided features, on occasion, but this shouldn't happen here because the velocity dispersion of the group is so low - any anyway there are very few other group members. Low velocity encounters can also do it, but it takes a long time for the system to evolve to that point - and the clouds remain suspiciously close to their assumed parent galaxy.

So I have no idea how these things formed, and that's why I like 'em.
https://arxiv.org/abs/1802.05279