Ten Dark Galaxies That Are Way More Awesome Than Dragonfly 44

.... no, not really. Well maybe not. Thing is, it's complicated.

Dragonfly 44 is this very faint dark matter dominated galaxy that's currently getting a lot of media attention. And rightly so, because it is very strange. At least it might be. But we don't yet know for sure just how strange it is : if it's as massive as has been directly measured, it's not that weird. However if it's as massive as the estimates suggest, then it's really very strange indeed.

As I show in this post, we already knew of a lot of optically very faint objects that are very hard to explain. We don't know how gas gets into the dark matter halos in the first place, and we definitely don't know how it turns into stars. So Dragonfly 44 doesn't break those aspects of galaxy formation theory, because they were already in a pretty bad shape anyway.

But what cosmological models predicted was that there should be a lot more very small galaxies than we'd previously detected. Recent discoveries looked like they were beginning to find them... not in sufficient numbers, but enough to hope that maybe the theory wasn't so bad after all. Dragonfly 44 might throw a spanner in the works, if its mass is as high as the estimate suggest. There was never much of a problem with the theory for galaxies of this mass, it seemed to be in decent agreement with the observations.

So Dragonfly 44, together with a bunch of other observations, might indicate that there are a whole load of giant dark galaxies that were never predicted by the theory. That could be a big problem. But I'd reserve judgement for the moment, for two reasons. Firstly, we don't have many good mass estimate for these new galaxies - and those we do have suggest the majority are dwarves after all. Second, the high mass estimate for Dragonfly 44 is a huge extrapolation based on the known-to-be-flawed numerical simulations.

What's the answer ? I have no idea. It's not the sort of thing that can be answered in a blog post, it's the kind of thing you need many different people to look at for the next several years. Right now we don't even really know the nature of the problem. It could be potentially very exciting, but it's just far too early to tell.
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