No, really, it's supposed to do that

For once a simulation where we expect everything to disintegrate....

So there are these weird hydrogen clouds in the Virgo cluster without any stars. For years, people have been waving their hands and shouting, "tidal debris !", meaning they think they were ripped out of galaxies during close encounters. This is almost certainly the case for some of them, but, as I shall show in a forthcoming post, it cannot be true for all of them.

The weirdest clouds have line widths comparable to giant galaxies. A line width just means we've measured how fast the gas is moving within the galaxy along our line of sight. If we had very high resolution we'd know if the gas was rotating or not, but these clouds are too small to do that easily. So we just know how fast all their gas is moving along our line of sight. And it's too fast. You just can't produce structures like this in tidal encounters, and we'd expect the clouds to quickly explode if they were only composed of the observable gas - so quickly that it's not very likely we'd ever observe them.

But then Burkhart & Loeb (http://adsabs.harvard.edu/abs/2016ApJ...824L...7B )came up with another idea : maybe the clouds are pressure confined by the intracluster medium. Even intragalactic space is not empty, and although it's very thin, the gas inside clusters is also very hot. So the pressure from this gas could prevent these clouds from flying apart.

Now, if the clouds high velocity width was just due to their temperature, that might work. The pressure from their heat could neatly balance the temperature of the external gas. The problem is that the temperature required (>100,000 K) is much too hot for the gas to remain neutral - it should be ionized. The only way out is if the line width arises from turbulent motions instead. But turbulence, by definition, is unstable.

So here's the first result of a little project to investigate how long these clouds could last if they were the turbulent-supported spheres proposed by Burkhart & Loeb. The answer ? Not long. As you can see (same sim in both gifs but with a different colour scheme) the clouds almost instantly disintegrate, but what you can't see is that they rapidly heat up. In about 50 million years all of the gas would be ionized, and they'd become undetectable in rather less than that (quantifying how much is a work in progress). That's far too short to explain how the clouds got so far away from the nearest galaxies, and it's difficult to see how they could even have formed in the first place.

But this is a teaser. More to come.
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