Chained in darkness

This is a very interesting paper about my favourite topic, optically dark gas clouds.

As far as I can tell, the discovery is purely serendipitous. They pointed the shiny new MeerKAT telescope at a galaxy group as part of a larger survey, and happened to find this linear chain of seven clouds. Hydrogen clouds without optical counterparts are pretty rare things, but these ones are among the most unusual. They're spread over an enormous extent (400 kpc !), are each pretty massive (around a billion solar masses, roughly the same gas mass as a chunky dwarf galaxy), have high line widths (~130 km/s, again dwarf galaxy territory), and have no obvious association with anything optical.

Somewhat oddly, they begin with a quite protracted introduction discussing why HI clouds are in general unlikely to be dark galaxies. While I agree, they erroneously cite my papers as supporting this, which of course they don't - rather, we found that certain objects are more likely to be dark galaxies than others. Even so, the introduction is a nice overview, though it feels a bit strange that they don't much discuss the possibility later on.

The paper is largely a discovery and description affair, avoiding much speculation as to the origin and nature of the objects. This is very sensible. From what we found, objects with high line widths, say > 50 km/s, are very difficult to explain as tidal debris in clusters. But these objects aren't in clusters, they're in a small-ish galaxy group (18 members), where the speeds may well change the kinematics of any HI clouds produced in galaxy-galaxy interactions. It's also impossible to ignore the neatly linear alignment of the clouds and the fact that part of the complex forms a sort of V-shaped feature. So debris of some kind does seem to be at least a plausible explanation.

Unfortunately they don't provide optical imaging over a large area, so it's difficult to see which if any galaxies these might relate to. But within the area they examine, there's no galaxy which looks like an obvious source of the gas. It reminds me quite a lot of the Kent Complex, which sadly they don't present as a comparison object (though they do compare it with HI1225+01, which is also similar). And there are certainly no big nearby galaxies with HI tails that one could point to and say, "there, that's the bugger !".

It also seems to resemble the Leo Ring in some aspects, which has comparable mass, size, lack of optical counterparts, and environment. But the kinematics and morphology of the Ring are very different. Although it lives in a different environment (a rich cluster), and measures only about a quarter of the size of this feature, to my mind the Kent Complex is overall more similar : comparable in mass, linear-ish morphology, and with several distinct "clumps" of similar kinematics. The latter is especially important, since line widths this high are extremely unusual.

No other feature is a perfect match though, and importantly, the origin of the Kent feature also remains a mystery. Dynamically, the major source in the MeerKAT object would require some dark matter for it to be stable, though not nearly as much as a typical galaxy. So neither "dark galaxy" nor "tidal debris" is wholly satisfactory in either case. Surely, it can't be a coincidence that this feature is so close to a galaxy group, but what's especially strange is that the group is HI-poor. How do you remove so much gas - nigh-on ten billion solar masses - without much affecting the optical component of the galaxies ? How do you remove the gas so completely from the parent galaxy ? At least some ought to be left in the galaxy itself, deep in the potential well.

All in all, a very interesting read about a fascinating object. Unusually, further observations here require not better spatial resolution (they already have that) but better kinematics - a resolution of 45 km/s is pretty awful, really. Even so, it's not at all obvious if better velocity resolution would help. My bet is that it will remain mysterious for quite some time to come.