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 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.
I'm in slightly over my head over here, but I have to ask: why is it 'spiky'?
ReplyDeleteIf we considered it as two gasses of different temperatures and densities, then wouldn't they curl up like smoke-balls and like?
Matter Beam Well, that's complicated. The structure will depend on the velocity gradient of the turbulence - if the velocities vary strongly over small distances, you'll get a different structure than if the gradient is shallower. Plus all the gas has significant self-gravity : it wants to collapse. Simultaneously it's being heated by by the surrounding hotter gas, which makes it expand. Unlike fire though, there's no source of energy to maintain the turbulence, so its continuously being resisted - only the initial energy allows it to keep moving. Also unlike fire-borne smoke, the dense gas here starts off stationary with respect to the intracluster gas - if we put a wind in, things might change significantly. Finally, the structures seen depend to some extent on the resolution of the simulations, which here are rather low to keep things fast.
ReplyDeleteThe short answer is that there are really too many variables to intuitively predict the shape of the gas.
Rhys Taylor I see. Gut feeling has no place here.
ReplyDelete