Accept no imitations

So I've gone from being swamped with writing a grant application to being swamped with writing an observing proposal. Yay. But I had to read this paper, because a) it's just a letter so it's nice and short and b) it's all my favourite topics mashed together.

This paper looks at a remarkable object in a compact group of galaxies. There are several big, dramatic galaxies present, with shell-like structures and arms and stars flying everywhere, but that's not what the authors are interested in. Instead, they pick on an innocuous-looking blue blob, a structureless starball peeping out from behind one of the bigger beasts. It was previously noted as being at the end of an HI tail, so the obvious idea is that it's a tidal dwarf galaxy formed from of the torn-out gas from some galaxy-galaxy interaction.

Tidal dwarf galaxies are pretty interesting in themselves, but the properties of this one overlap with not one but two other interesting objects. I guess that cubes it interestingness, making it so interesting that everyone had better had a lie down lest they get too excited.

While the smooth, structureless nature of the object isn't anything special, its stellar population seems to be wholly young : 60 Myr or less. Most "normal" galaxies have at least some very old stellar component at least a few gigayears old, even those that are dominated by a younger crowd. And this little galaxy doesn't have many stars either : so much so that it easily counts as a Ultra Diffuse Galaxy.

"Ahah !" you may be thinking. "That means UDGs are probably just tidal after all, so we can put all this hoo-hah about whether they're massive, dark-matter dominated galaxies to rest and get on with our lives."

In fact you can't. For one, UDGs are diverse enough that we know we can't paint them all with the same brush anyway. For another, this particular tidal UDG appears to be dark matter dominated, by a factor 3-10. And that's not at all expected for tidal dwarf galaxies, which are supposed to be made of purely baryonic matter.

And it gets stranger still. Given that there appears to be no ongoing star formation this object, based on UV data, the authors calculate that it will become totally invisible in about 2 Gyr. So although it's merely a faint galaxy right now, eventually it will become a truly dark galaxy. The implication, of course, is that many other such objects could already have been produced, so they're floating freely around the place confusing all kinds of hapless radio astronomers. See, there's been lots of speculation about how HI clouds can appear to masquerade as dark galaxies, where the dynamics indicates a lot of extra mass present that isn't actually there. But this object shows that there could be some such "fake" dark galaxies which really are... dark... galaxies...

This is all a lot like the idea of faking the Moon landing on the actual Moon. Or if you want a more colourful philosophical conundrum, boobs. Are they still real if they're fake ? No, don't answer that.

What they really mean is that the idea of dark galaxies is that they're supposed to explain the missing satellite problem, wherein too few galaxies are observed compared to simulations. If most of them are just too faint to see, problem solved. These would be primordial dark galaxies. But now this little object suggests we may have genuine dark galaxies produced by an entirely different, much later mechanism that would have nothing to do with the cosmological difficulties at all. So that's an extra large spanner thrown in the works. Yay.

Personally I'm not entirely sure how secure their mass estimate really is. The line width of the object is quite narrow and the inclination correction must be difficult; the HI and optical centres are offset. I also don't understand their statement that the object could become self-bound even without dark matter : they say there's a mass threshold for this, but this must surely be dependent on other factors too.  And I don't understand how you could get such large amounts of dark matter in a tidal dwarf anyway; in simulations, it tends to disperse very easily. So this object potentially raises a lot more questions than it solves - as all good discoveries should. Hurrah !

A diffuse tidal dwarf galaxy destined to fade out as a "dark galaxy"

Assuming that the object is dynamically stable and able to survive in the future, its fading in time via the aging of its stellar component will make it undetectable in optical observations in just ∼2 Gyr of evolution, even in the deepest current or future optical surveys. Its high HI mass and future undetectable stellar component will make the object match the observational properties of dark galaxies, that is, dark matter halos that failed to turn gas into stars. Our work presents further observational evidence of the feasibility of HI tidal features becoming fake dark galaxies.