Congratulations Dr Taylor, it's a disc !

More FLASH experiments. Stage 1 (produce a stable gas disc that sits there, quietly minding its own business) is almost complete. Here it is after a billion years (5 rotations) - it resolutely refuses to disintegrate like the last one did. Since it doesn't do a lot, this one is just a rotation movie of the last output frame from the simulation.

To make things more interesting, the transparency here is equivalent to density, while the colour shows temperature. I used a logarithmic scale for the density, so the hot gas around the disc is actually much, much less dense than it looks here. That's why the inner part of the cold disc appears to be hidden. It would probably be quite interesting to watch the temperature evolve with time, but this is just a crude hack so that would take a bit longer to code.

The reason the last disc went bbblewwwerrrgh (technical term) is probably because FLASH has problems with certain boundary conditions. The mass of gas increased tenfold over the simulation, and the disc just gave up. This time, using different boundary conditions (the space is a hypertorus, so anything that hits one edge magically re-appears on the opposite edge), mass is conserved, and everything is basically tickety-boo. Apart from a density increase in the center, which is slightly annoying.

Stage 1 is our control experiment. When that's done, we'll be able to chuck other galaxies at it and see what happens, and who knows what else. We've got to be sure that the disc is stable if left alone, otherwise our interacting galaxy simulations wouldn't tell us very much.