Ram Pressure Stripping Made Slightly Easier Than Before

...Or Actually Very Easy Indeed If You Use These Handy Online Tools

This paper was in development for an outrageously long time and all the hard work was done by the lead author, who intends it to be his final paper as first author. I only came in for the last few years, to give you some idea of the timescale involved.

Ram pressure stripping is a process which affects galaxies moving through an external medium - usually the hot, thin gas in a cluster. The motion of the galaxy causes the external gas to pile up in front of it, generating a "ram pressure" which can be sufficient to remove gas from the galaxy. Unlike tidal encounters, the faster the galaxy moves, the stronger the effect. This makes it extremely important in clusters where galaxies tend to have extremely high velocities - great for ram pressure stripping, lousy for tidal encounters. The particularly high density of cluster gas means that ram pressure can become strong enough to completely remove the gas even from a giant galaxy, while leaving the stars essentially unaffected. It's thought to be the dominant process affecting the evolution of gas-rich galaxies in clusters.

Calculating exactly what effect this has was until now a choice of either using a single, very crude analytical formula, or arbitrarily complicated numerical simulations. Here we present a middle ground - a series of equations describing all the effects of ram pressure under just about every conceivable situation, but, much more usefully, interactive online tools for running simple simulations in a web browser. The full set are available here. But in particular see this one, which lets you control a simple particle simulation of RPS (with sensible default values to get you started).

(and no, I actually don't care if you hate JavaScript, go suck a lemon - I didn't write these anyway)

And it all seems to work pretty well. Results are in good agreement with observations and simulations alike. This makes it possible to calculate quite specific effects of RPS on large, statistical samples of galaxies. For example you can estimate how long a galaxy has been experiencing significant ram pressure and how long you expect its stripped gas tail to be, or how much of its displaced gas it should eventually re-accrete as it falls back to its disc.

Of course this does require some assumptions about the motion through the cluster and the density of the surrounding material etc. so there are plenty of uncertainties, but that's true of more complex simulations as well - and these ones are thousands of times faster, if not more. What they can't handle are galaxies moving edge-on through the gas. For those cases the complex fluid physics of two interacting gases becomes more important, so it doesn't work very well. But in other situations it seems to do a very good job. My next paper will be about observations of galaxies with long stripped tails which the model has successfully predicted.
https://arxiv.org/abs/1806.05887