At least that's what I've concluded. After much tweaking, this is the best I could come up with. It's sooo close... and yet so far. The disc manages to make about a third of a rotation and it looks good... and then it collapses.
The problem is that in order to prevent fragmentation of the disc it needs to be hot. But if it's too hot it simply expands to infinity. According to some quick and dirty calculations I did this afternoon, there's no sweet spot - if it's hot enough to avoid fragmentation, it's also hot enough to evaporate.
Still, it's close - there's just one big fragment produced, while the outer parts slowly evaporate. Only about 1/3rd of the original particles remain within their starting radius by the end of the simulation, while the core is many times denser than the original disc.
Maybe if I change the parameters enough from my original requirements I could make this work, but it's not worth the effort. Uniform pure gas discs aren't found in nature anyway. Tomorrow I'll try adding a dark matter halo. Having a lot of extra mass near the centre should stabilise things much more easily - the dark matter should be much less prone to fragmenting, and the extra mass will keep everything bound so the gas can be hotter. At least that's the theory...
It would not be a problem to put them all on one page, when experiment ends?
ReplyDeletethey come and they go...
ReplyDeleteThis sounds like you have a fair match with observation up to a significant cloud size.
ReplyDeleteIs there any reason to expect a contrary result on the scale you are simulating?
how about ionization fraction? Radiation interactions? Magnetic fields? Pretty close if those things are left out.
ReplyDeleteIvan Petkovic The blog post is in preparation. :)
ReplyDeleteEveryone else : true, this could be solved with enough physics. The simplest approach would be to vary the density so that there's more mass in the centre. As you can see at the end, that situation is stable.
At this point, this is mainly an intellectual exercise to see if it's possible. Turns out it isn't, but I learned a lot from it. Gas discs are remarkably unstable objects : there's the ring instability of the previous runs, the Jeans instability, the Toomre instability (which causes them to fragment into spinning chunks because of differential rotation)... and that's all just from gas, gravity, and motion. Clearly, taking an incremental approach is a good idea !
The underlying goal is to simulate what would happen to a gas disc with a dark matter halo if it fell into galaxy cluster. There are several known hydrogen clouds floating around that can't readily be explained as being pulled out of galaxies, so one hypothesis is that these are "dark galaxies". If they can survive in the cluster environment, then this would be an interesting result. If they can't, then that's equally interesting because the things are darn hard to explain (for reasons I shall go into in a forthcoming post).
For the current study I'm just using gas, dark matter, and gravity. Nothing else. Even with that limited scope the parameter space is huge. So the plan is to add more physics incrementally - but on a timescale of years rather than days.
This sounds like a job for machine learning!
ReplyDeleteThe goal function being too maintain a gas disc for as long as possible with initial parameters.
Then allow it to adjust the variables so as to maintain it for as long as possible, e.g. allow it to add/remove heat from the system over time.
Oliver Hamilton Not necessarily, as the code author writes :
ReplyDelete"I spent about 6 months of my PhD on it and they still weren't great. And its not just me - DICE is a very clever parallelised code used for setting up galaxies. It runs on 8 cores, chugging away for half an hour, carefully measuring the potential gradients everywhere on logarithmic grids. Then you run the initial conditions and the disk collapses, bars, forms rings, etc, etc!"
Another thing I realised after some discussions is that there's a huge pressure gradient at the edge of the disc because there's no external medium. That's probably another reason why the denser gas in the centre remains stable - the gas there has pressure confinement. I could make the gas disc extend a whole lot further, which might improve things... so much physics in such a "simple" system !