Just what the hell are High Velocity Clouds, anyway ?

Some interesting papers that were brought to my attention in a group meeting yesterday. I normally apply strong mental filters to anything talking about star formation since stars are nothing but a waste of precious neutral hydrogen, but in this case I'll make an exception.

Around the Milky Way there are several hundred (perhaps thousands) of known clouds of hydrogen. Many of these appear to be well outside the disc of the Galaxy and most are moving much faster than gas in the disc at the same distance. These "High Velocity Clouds" (these are people who think 'Very Large Telescope' is a sensible name, after all) have been something of a mystery for decades and have everyone well and truly baffled. Here are seven weird reasons why we finally know what they are...

Or not, because it never ever ever ever EVER works like that, you muppet. In fact, several explanations have been put forward. They could have been blasted out of the Galaxy by multiple supernovae exploding in star clusters, eventually falling back to the disc through gravity. Or they could be "dark galaxies" that haven't yet formed any stars. One problem is that we don't have a good idea how far away the clouds are, so we can't properly estimate their size and mass. There are lots of ways of estimating the distance to stars (depending on the details of the situation), but hardly any when there's just pure gas, as the HVCs appear to be.

This first paper...
http://arxiv.org/abs/1501.03707
... claims the detection of star clusters inside one of these HVCs. Based on the stellar content of the clusters they estimate the distance at around 8-10 kpc (26,000 - 33,000 light years). That's far enough away that the "dark galaxies" idea becomes plausible - if they were a result of the supernova-driven "galactic fountain" model, they probably couldn't reach such large distances. They also show (though rather less convincingly) that the orbits of the clouds are compatible with them falling toward the Milky Way from a large distance, though the orbit determination is not very robust. The number of stars in the clusters is also not large (a few tens), so I'm not sure how accurate any of this really is.

Then again, I don't find this...
http://adsabs.harvard.edu/abs/2016A%26A...589A.123K
... rebuttal paper all that convincing either. They make a lot of different arguments why the clouds can't be so far away, but none of their ideas seems particularly solid to me. Two examples :
1) The hydrogen emission of the clouds appears to be neatly correlated with a drop in the X-ray brightness in that region, which can be explained if the clouds were embedded in the hot X-ray emitting gas of our Galaxy. Problem is I'm not sure anyone has a good idea of how far this hot gas extends.
2) From another paper they find that the reddening of background sources (http://astronomy.swin.edu.au/cosmos/I/Interstellar+Reddening) appears to reach a maximum at only 400 pc (1,300 light years). If the clouds were really 8 kpc away, there should be another increase in reddening at 8 kpc. But there are lots of problems with this idea : the reddening data shows a very strong scatter and the cloud is faint, and since they don't quantify how much reddening there should be, I'm very skeptical that this would be detectable.

Another paper...
http://arxiv.org/abs/1607.00672
... by the same authors of the first, claims that now they've found a few more star clusters in some other clouds. This paper is still under review and as it stands I wouldn't accept it - it has so few details that it's basically useless. It's potentially very interesting but at the moment it doesn't have enough information for anyone to properly judge it.

So the mystery of the HVCs continues. We've gone from a case of, "whatever they are they definitely don't have any stars at all, absolutely definitely none" to, "well, maybe some of them do form a few stars". The jury's still out on whether we're just seeing our Galaxy farting clouds of gas into the cosmos or we're actually surrounded by a swarm of dark galaxies beginning to rain death and destruction* down upon us as they hurtle toward the Galactic disc.

* A very small amount of gas and some minor disruption.

If we get more details in the third paper, this could become extremely interesting. There are no boring solutions here. Either our Galaxy is an incredibly messy, violent place (which still leaves us with the unexplained lack of small galaxies nearby), or cosmological models were right all along except that we have no idea how gas gets into dark matter halos. Still, it looks very much as though the poo-poohed idea of dark galaxies is enjoying an increasingly vigorous renaissance. More research is needed.
http://arxiv.org/abs/1501.03707