My hydrogen is better than your hydrogen

There's hydrogen and then there's hydrogen. But which hydrogen is the best hydrogen ?

In the last few years, the prevailing view has been that it's molecular hydrogen (H2) that's important for star formation, with atomic hydrogen (HI) being a sort of boring sideshow. HI is like the thousands of hapless contestants on The X Factor who have all the singing talent of a diseased cat that's being run over by a lawnmower, whereas H2 is like the incredibly small number who can actually carry a tune. Every H2 molecule originally starts out as two hydrogen atoms, but only when they combine to form the molecule does star formation have any chance of happening. At least, that's the theory.

This paper challenges that in an unexpected way : by looking at galaxies which host gamma ray bursts. GRBs are thought to be the result of massive exploding stars. The authors measure the HI content of the host galaxies for the first time and find that they're somewhat gas rich. Not like, OMFG that's gassier than that bloke in the pub who farted a tune, just a little bit more HI gas than normal. Whereas in general GRB host galaxies are known to have very little H2*. The star formation rates and other properties of these galaxies are also entirely normal. How can they have normal star formation rates, host GRBs and yet not have much in the way of H2 ?

* A major weakness is that they only have H2 (indirect) observations for one galaxy, and that only gives an upper limit - which is not particularly low. Their conclusions would be much stronger if they had observations of these particular galaxies, but for now they have to assume the galaxies are typical of GRB hosts in general.

The authors suggest four possibilities :
- The UV radiation from the massive stars which are forming might dissipate the H2. But that would be unusual, it should only happen very close to the most massive stars - not throughout the entire galaxy.
- There might be more H2 than is detected. This is a possibility I think they don't give enough credit to - H2 can (usually) only be detected indirectly (silent but deadly, if you will), so it's possible there's a lot of undetected H2. Jury's out on that one.
- The HI might be rapidly converted into H2 which is then very, very efficiently converted into stars. That doesn't work so well since the formation time of the H2 is thought to be much longer than the collapse time of the HI.

Which leaves the possibility that in this case it's the HI that's forming the stars directly. And why not ? They have plenty of HI but not much H2 at all. Theoretically this make sense.

H2 is thought to require dust grains on which to form, but GRBs have been shown to occur in very dust-poor regions of the host galaxies where there shouldn't be much H2. And if stars are forming from pure HI, it makes sense that they'd be particularly massive and so lead to GRBs : HI is warmer than H2, and therefore needs more mass to collapse. H2 is cold and so is much more prone to fragmenting - it has less thermal pressure pushing it outwards. Also the observed correlation between star formation rate and gas content is tighter when the HI is included.

One of the really neat things is that one of the host galaxies they study has a massive, optically dark cloud of HI very nearby. It's almost as massive as the galaxy, about the same size (and so density, which is the important thing for star formation) and a similar line width (which may indicate rotation). This is extremely strange and it's surprising they don't comment on this further - this is one of the most massive optically dark HI clouds known, and its line width is respectably high. They interpret it as a signature of cold accretion : primordial gas that's flowing from intergalactic space into the galaxy (though they say higher resolution observations would be nice).

I've never liked this explanation. Why does this appear to be happening only in a handful of galaxies ? Why not around every isolated galaxy ? And why is the part of the HI close to the galaxy so incredibly dense - why isn't there a longer, more diffuse tail ? Why are these clouds usually seen on only one side of the galaxy ? Why isn't it forming stars ? It can't be the product of a tidal encounter because there's nothing nearby to have an encounter with, so what's going on ? Are those my feet ?

One proton, one election - lots of unanswered questions.
http://adsabs.harvard.edu/abs/2015A%26A...582A..78M