An Ionised Gas Blob

This is an interesting little paper about a possible "dark galaxy" candidate. It contains the memorable line :

""In general it is very difficult to identify dark galaxies since they are extremely faint in the optical."

Yes, well, the thing about dark galaxies is....

Anyway, this team looked at this nice little group of galaxies and found there's a strange blob of warm ionised gas near one of the elliptical galaxies without anything visible at optical wavelengths. So it seems to be a star-free cloud of gas. And the velocity dispersion of the gas is quite high, implying quite a high dark matter content. Adorably, they've even given the blob a name (Totoro), which I think is a practise everyone should adopt.

But there are a number of problems with this. First, the cloud is so near the elliptical that it could be hard to see anything behind the galaxy. Second, there's another galaxy very close to the centre of the ionised blob - not exactly coincident with it but enough to make me go "hmmm". Third, although the ionised gas mass is pretty low, they only have upper limits on the atomic gas - potentially there's enough to greatly reduce the required dark matter content. Fourth, Totoro isn't completely detached from the elliptical galaxy - it's connected directly to the centre by at least one (and perhaps two) gas streams.

Fifth, we've shown extensively that small clouds with high velocity dispersions - embedded in streams - can be produced by interactions. Getting them to become separated from the streams is very, very hard indeed, but within streams they're common features in our simulations. The team here performed their own simulations to try and simulate Totoro, but these only included stars and virtually no other details are given - they don't even say how many simulations are given, let alone how many particles they used.

Another scenario they consider is that Totoro is the result of gas ejected by an active galactic nucleus in the elliptical galaxy. That ties in well with the stream connecting the two objects, and the complicated velocity structure of Totoro. Personally I think this is the most likely explanation. Although most elliptical galaxies don't have much gas, some do - occasionally large amounts of it.

I think it would be really difficult to pin down what's going on here. The group of galaxies is a very complex environment even just considering the gravitational fields. Add an AGN into the mix and things get really tricky. There isn't really anything inconsistent with it being a dark galaxy, but I think it would be very difficult indeed to establish that the other explanations don't work.
http://adsabs.harvard.edu/abs/2017ApJ...837...32L