The other paper I read today on the train. This one's about an almost dark hydrogen cloud in the Virgo cluster, a.k.a. my personal obsession.

This particular cloud has been detected some time ago, but this new analysis adds more observations and simulations. It's got about 30 million solar masses of hydrogen and maybe as many as 100,000 solar masses of stars, although no-one is quite sure exactly. In any case, it's got way more gas than stars. Earlier papers hinted that it might be rotating, but very slowly. It's also very compact, perhaps only 1/30th the size of the Milky Way. And it's really far from any other galaxies, so it's not easy to see how the little blighter formed.

Although the thing is so bloody faint it's hard to be entirely sure, the authors found no evidence of an old stellar population : stars don't seem to have started forming until about 50 Myr ago. Which would make it an incredibly young galaxy, and that would fit quite well with its overall properties (mass, size, velocity dispersion). But an old stellar population would be hard to detect, so the limits on that seem to be pretty crappy really.

More secure are the author's measurements of the cloud's chemical composition. If this really was a young galaxy, it should be nearly pristine, primordial gas. But it isn't - it's got the composition of a far more massive galaxy. With this few stars, it's impossible to produce such a composition in the 50 Myr time frame. What this means, the author's say, is that it's far more likely this cloud was ripped out of a galaxy.

But, given the distance of the nearest galaxy this cloud could have originated in, this means the itty-bitty thing would have to survive a billion years of travel through the hot, oppressive intracluster gas. Can it ? Their simulations say yes, quite happily - the intracluster gas actually helps confine it and prevents it from flying apart. As you may have seen (, we're running our own simulations of such an idea in Prague, with blackjack, and hookers... but we get a totally different result. The reason seems to be that the clouds we're interested in have a much greater velocity dispersion, so blast themselves apart in a great big splooch. The cloud in this paper doesn't have that problem : its thermal pressure can balance out nicely with the surrounding gas pressure, and it stays as a happy little gas cloud for a billion years or more.

All well and good, but of course the mystery isn't solved yet. As the authors note, it's difficult to explain why the cloud should have gone along quietly minding its own business for a billion years and then VERY SUDDENLY DECIDED IT WANTED TO FORM STARS for some reason. It's not near any galaxies today, so tidal encounters can't explain it. While clouds like this one have been produced in lots of different studies, they're always part of a much larger star-forming "wake" of stripped material. In this case, that material is completely absent. Where's it gone ? Why did this lonely little guy survive, all alone on the wide wide sea ?

No-one knows. But if the bloomin' Arecibo Time Allocation Committee would just bloomin' well realise the value of big surveys, we could detect a whole bunch more of these critters and maybe start to figure something out.
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