A huge hydrogen stream in an galaxy group no-one's ever heard of

Very interesting paper today about the discovery of an enormous, 500 kpc (about 1.6 million light years) hydrogen stream in an otherwise obscure galaxy group. Earlier observations with ATCA (Australian Telescope Compact Array) had found something, but observations with the shiny new Karoo Array Telescope (KAT-7) revealed a much larger, fainter structure. KAT-7 is only a precursor to the larger MeerKAT, which is itself a prototype for the even larger Square Kilometre Array, giving some indication of what we'll find with the next generation of radio telescopes.

For comparison, as far as extended gas goes Arecibo is about ten times more sensitive than KAT-7, which is itself about ten times more sensitive than the VLA. The other advantage of KAT-7 and other SKA "pathfinder" instruments is that the field of view is much larger than in the older facilities. Getting that sensitivity increased by another factor of 10, though, is a formidable challenge indeed, and new instrumentation on Arecibo could expand its field of view by a factor of a few. The VLA also possesses superior resolution. In short, radio instrumentation is fiendishly complicated, and anyone who thinks it isn't does not know what they're talking about.

Anywho, this new giant gas stream (see their figure 2) is in a fairly small galaxy group. In galaxy clusters, the main gas loss mechanism is believed to be ram pressure stripping, where galaxies move through the hot, thin surrounding gas. That can't really be the explanation here, since a) there isn't much in the way of external gas; b) the galaxies are moving too slowly. Which means the most likely explanation is that the galaxies are gravitationally interacting. Unlike ram pressure, that's actually more effective because of the low velocities, since it gives the gravitational forces more time to act.

This is sort of plausible for this case, but there are some interesting oddities. Their figure 4 shows higher resolution (but lower sensitivity) observations with the VLA of the likely parent galaxy of the stream. It does seem to be interacting with a nearby companion, in that they both have one-sided gaseous extensions. But normally such interactions produce two tails, characteristically on opposite sides. That doesn't seem to be the case here. The authors say there's a twin-tail structure, but I don't see it.

A lot of the other galaxies in the group also seem to be interacting. Taken together they form a single, coherent, giant structure. The velocity of the gas shows pretty convincingly (figure 3) that this is likely a single entity, not a chance alignment of different features. I suppose that's possible though, if the galaxies were all falling into the group along a filament. It also shows a strange bifurcation, with one part of the stream at one velocity but (at the same spatial location) other parts are at quite different velocities. And the velocity changes look to be pretty sharp. They also say that there's a cloud nearby with no optical counterpart, though it isn't clear to me which feature they mean by this or its velocity gradient.

In context this is especially interesting to me because we've shown how it's possible to make this "kinky" velocity structures in galaxy clusters (http://astrorhysy.blogspot.cz/2017/01/check-out-my-kinky-curves.html). But clusters are much more massive than groups, so I wouldn't necessarily expect the same effects in groups. Naively, I would expect the lower speed of the interactions to be better at drawing out long gas streams, but worse at causing kinks. Only more simulations will answer that one.
https://arxiv.org/abs/1803.08263