A new explanation for optically dark gas clouds

I am not at all biased in any way that this paper discusses some of the optically dark hydrogen clouds found as part of my thesis project. I care not a jot than an Einstein fellow and a Harvard professor have decided that these objects are worthy of investigation. I am definitely being totally objective in choosing to report this paper.

What the authors do here is to look at the hot intracluster medium in the Virgo cluster (detected through X-rays) and the optically dark hydrogen clouds detected through radio surveys. They analytically predict the sizes of the hydrogen clouds if they were in pressure equilibrium with the hot gas (that is, not expanding or contracting). Why should they be in equilibrium ? Well, if they weren't, they'd either expand and become undetectable, or collapse and form stars. Both process would be quick, so it's not likely that we're detecting them in this narrow interval. If they're stable, they'd be long-lived and therefore it's much more likely that we'd detect them.

To be in equilibrium, they find that the true sizes of the clouds would be compatible with the observations. Currently we only know the maximum sizes of the clouds because Arecibo's resolution isn't good enough to determine their true extent. They also find that the clouds would be just below the threshold at which they should start forming stars. There's a bias towards detecting clouds near this threshold because if they had any less hydrogen they'd be undetectable - so there could be more small clouds we haven't detected.

What they don't discuss, which you'll have seen recently if you follow my Astronomy Graphics collection [now defunct due to the end of Google Plus], is the origin of the high velocity width of the clouds. As I've shown through extensive simulations over the last year, this is damn difficult to explain through tidal encounters, but easy if the clouds have their own dark matter halos (exhaustive 27-page paper to be submitted next week). I'm not sure what the pressure equilibrium condition means for the dark galaxy scenario - probably that turbulence in the hot gas is another interpretation of their high velocity widths. On the other hand I don't see that this necessarily precludes the possibility of a dark matter halo.

In any case, they emphasise the need for follow-up observations, which ought to make things easier come the next observing proposal deadline.
http://arxiv.org/abs/1604.01767