Rhys Taylor's home page
Science, Art and Data Visualisation

Galaxy Simulations

These projects generally use the techniques outlined in the n-body section, though in a few cases I've also animated simulated gridded FITS data using FRELLED. Most of the n-body stuff comes from Rory Smith. Back during the Cardiff days, we were both using an SPH code called "gf", and the in-house visualisation code of 2008 was... crude. It could show you particle positions from three different angles and that was it. Actually this was how I started learning Python to begin with, so that I could show the particles in Blender and make them look prettier. This is by no means a complete collection, just some highlights.

These simulations are nothing terribly sophisticated, mostly run on desktop machines from ~15 years ago.

From disc to sphere

One of Rory's neat findings was what happens to dwarf galaxies if they experience ram pressure stripping. One idea is that dwarf irregular galaxies entering clusters lose all their gas, stop forming stars and then eventually transform into red dwarf ellipticals. But this was controversial because the morphologies of the two are so different : irregulars are complex, highly-structured discs, whereas ellipticals are basically smooth spheroids, and they have different light profiles. It isn't obvious how simply removing the gas could cause a change like this, at least not rapidly. But Rory's simulation shows that removing the gas can directly change the morphology, not just but quenching the star formation. In dwarf irregulars the mass of gas is a significant fraction of the total, so yanking this out rapidly exerts a strong, protracted gravitational pull on the stars, turning the disc into a spheroid.

From disc to wheel

Another simulation Rory ran (just for fun, as far as I remember) was what would happen if an elliptical galaxy collided head-on with a spiral. The idea was that the changing gravitational forces could produce something like the famous Cartwheel Galaxy, and indeed this is just what happens. I also visualised a simulation Rory ran some years later : the formation of a specific object, Auriga's Wheel, modelled in much more detail. There the agreement between model and observations is really very impressive.

Feel the force flow

A simple simulation using cubes to represent particles, coloured according to the total gravitational force they were experiencing. The idea was to try and understand how harassment works, whether we could help identify the specific galaxies responsible for the disturbances. The answer : not really, but it was fun to try. Presumably the visualisation process could be improved a lot these days !

Destroying a galaxy

Is dark matter really needed ? I've only slowly come around to the idea that it is, and back in my PhD days I was far less convinced. In fact my Master's project was all about simulating the formation of a galaxy without using dark matter, which, for various reasons, didn't work. At all. Still I'd written a short review of how dark matter didn't seem especially necessary, and I do sympathise with the view that it doesn't seem to do much except make galaxies spin faster. But when you look at it in detail and do things the hard way, you realise that this just isn't the case. It's really, really hard to avoid the need for dark matter, so I asked Rory to make this little simulation (which I visualised) showing what happens if dark matter were to be magically removed. After telling people for years that it would cause galaxies to explode, this simulations shows pretty much exactly that. More details on my blog.

Melting a galaxy

After having gone through the process of setting up an n-body galaxy for myself, for simulations which I eventually used in a few publications, for a brief period I also dabbled in the FLASH grid code. This is a rather different beast, and I never got anywhere with this (there's far too much observational data to be dealing with anyway), but this early mistake was an educational experience. The disc was set up according to well-known prescriptions but it slowly dissolves into an ungodly writhing horror. The reason turned out to be a bug in FLASH's boundary conditions : as material was supposed to be leaving the edges of the simulation, it was actually dragging in more mass somehow. By the end of the simulation the mass was ten times greater than it was supposed to be, a classic, "well there's your problem !" moment.