The faintest galaxy ?

Another one in a long list of weird hydrogen clouds that don't make much sense. This one is part of the Leo group, inside the huge Leo Ring which is about 650,000 light years across. How this formed isn't known for sure, but the best guess so far is that it was formed by the collision of two galaxies. Simulations have shown this is plausible, though I'm sure everyone is by now aware of the difference between consistency and evidence.

Anyway, this particular cloud, BST1047, is one small part of the Leo Ring. It was already known from the HI observations that this cloud is rather separate from the main Ring. It was also known that elsewhere in the Ring are several actively star-forming regions, well outside of the gas' likely parent galaxies. So you might think that this is just one more extragalactic star-forming blob - kinda interesting, but not especially new. And while this little dude isn't likely to win anyone a Nobel, it is weird.

The parameters of this blob are extreme. Its brightness per unit area ranks it a the faintest object ever discovered by visible light. The brightness profile is also strange. Rather than peaking in the centre and continuously declining (e.g. exponentially), as in normal galaxies, here the brightness is uniform over the central region and only declines beyond a certain distance. That's much more like a typical gas density profile than a stellar distribution. It's also quite blue and has UV emission, suggesting ongoing star formation, but the gas density appears much lower than expected if that's the case.

Although it's got a weird surface brightness profile, the shape of the object's stellar emissions is kinda boring. In the HI, however, it's weirder (see their figures 2 and 3), with two parallel tails on the same side of the object, pointing in the same direction. Galaxy encounters are complicated, but normally produce two tails on opposite sides pointing in opposite directions. So the gas structure is strange.

And the dynamics of the object are also confusing. It's got a small velocity gradient of about 16 km/s across it, which, if due to rotation, indicates it's got very little dark matter. In light of the recent furore about that other galaxy without dark matter, I'm surprised they don't make more of this. But they're careful to note that because of the object's faintness and the fact it's likely gravitationally interacting with other objects, it would be premature to jump to any conclusions here.

What is it ? Dunno. The most likely explanation is some sort of debris from the interacting galaxies, but we don't really know. Leo is a strange place that needs more HI and deep optical observations.
https://arxiv.org/abs/1807.11544