Attack of the Flying Snakes

So here it is, my sixth paper as full author, 27 pages of text, under construction for ~18 months, more than 200 simulations and with movies of all them. Detailed blog posts (it deserves two) will follow shortly, but here's the super-short version for lazy / marginally interested people.


There are some hydrogen clouds in the Virgo cluster without any stars. The nearest galaxies look undisturbed and show no signs of any extended hydrogen streams, and they're pretty far away from the clouds. Yet the most popular explanation is that the clouds are some form of "tidal debris", meaning that they were ripped out of galaxies as they passed close to each other. Generally speaking this is quite a sensible explanation : after all, the gas has got to come from somewhere.

The problem is that thanks to one or two previous simulations - which until now no-one had really bothered to check - this explanation has been used for almost all clouds, regardless of their properties. These particular clouds have high velocity widths, meaning they look like they're rotating. The tidal debris hypothesis is supposed to be able to explain this. Actually, our new set of simulations show that this is due to people over-interpreting the results. Our simulations are consistent with the previous ones, but show unequivocally that clouds with high velocity widths cannot possibly be explained as tidal debris.

We also tested the alternative hypothesis that the clouds could themselves be "dark galaxies" - rotating hydrogen discs embedded in dark matter halos. That scenario turns out to do a far, far better job of explaining the observations, and seems to tie in quite nicely with the newly-discovered "ultra diffuse galaxies" (which are very faint galaxies discovered in the Virgo cluster which do at least have some stars, just not very many).

Why do these stupid poxy gaseous anomalies matter ? Because "dark galaxies" were proposed to explain the missing satellite problem, the observation that there are far fewer small galaxies than predicted by simulations. This has been a major thorn in the side of cosmological models for the last 20 years or so.

Not that we should get carried away. We've shown that tidal debris definitely doesn't work, and dark galaxies do work. But a model which works is not the same as a model which is correct. Other explanations are possible and our simulations (like the previous ones) are missing a lot of important physics. The take-home message of the paper is that if you find a mysterious hydrogen cloud, hand-waving explanations about "tidal debris" are just not good enough.

More research is needed.
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