Rhys Taylor - Death of the Flying Snakes
In the last batch of simulations, we dropped a long gas stream into the gravitational potential of a cluster to see if it would get torn apart and produce features
resembling those seen in reality : small, optically dark hydrogen clouds without stars. It didn't. But it did turn the stream into some remarkably snake-like structures,
which was nice.
An obvious and (relatively) easy to fix problem with this is that we just ignored the parent galaxy where the gas came from in the first place. We showed that this
probably wouldn't make any difference, but of course it's better to test this. This way we can model not only what happens to the gas streams but how they form in
the first place, as well as what happens to the galaxy.
It's just possible that if the initial structure of the streams produced is very different to what we assumed in the previous model, we might get a very different outcome.
Also, if this does produce the dark hydrogen clouds we're after, we need to know if the galaxy still looks like a galaxy - if it gets smashed into something unlike anything
observed, that would falsify the model. And probably the most important feature is the stars : with stars included in the galaxy we can now test if the clouds produced would
really be optically dark or not. Previously we could only measure the properties of the gas and just assumed there wouldn't be any stars present.
A preliminary analysis shows that, as predicted, including the galaxy makes no difference for the dark clouds : they still form but they're still very, very rare - and that's
without checking if they'd be optically dark or not, so I expect them to be even rarer in this model. The galaxies get bashed around a bit, but about half of them suffer
no noticeable effects at all. The other half have some nice structures induced in them for a while, but nothing outlandish. They don't even lose a measurable amount of gas.
And, alas, there are no more snakes...
People have been suggesting that dark clouds are some form of "tidal debris" for years, but there are only two other papers explicitly showing this is a possibility.
Unfortunately those results have been over-interpreted. While large dark gas streams can be produced, small ones with properties like some of those observed are almost
impossible to produce in this way.
There's still a fair bit of work left to do with these purely gravitational models. This model uses a galaxy with properties typical of a spiral galaxy, whereas the previous
claims were based on one using a less massive spiral with a more gas that was more extended. So we need to do run a direct comparison to see if and how that influences
the results. We also need to measure the stellar content of the stripped gas. Previous models said this was very low, I suspect ours will show it to be a bit higher. Still,
it's very clear that the pure tidal debris scenario just doesn't work.
What is very much harder to predict is what would happen if we included the hot gas in the intracluster medium. In principle this is easy to do, in practise not so much.
More physics, more computationally expensive. The original "flying snakes" took just a few days of computation time. These galaxies - which have more components and more
stars - took about three weeks. Adding in the surrounding gas could make things take several times longer again. We'll get there eventually, but not today.
