is the set of non-negative, real numbers. A computer simulation is just a means of evaluating such a model, a simulation *run* being an evaluation for a single parameter. In galactic dynamics we are typically interested in simulating (i.e. modelling) observations of a galaxy.

When a model is computationally expensive to evaluate we may use a *meta-model*, i.e. a model of the model that is computationally cheaper. An *emulator* is a meta-model together with a measure of confidence in that meta-model’s output. In emulation we perform a small number of runs, each using a different parameter, and then use the resulting data to estimate the output for a parameter that we have not explicitly computed, along with a confidence interval for that estimate. We do this without needing to make an additional simulation run or model evaluation. In this respect, emulation is a matter of *prediction*. But, since we may make a prediction for all points in the parameter space, we can fully map out the function being emulated and in this respect emulation is an efficient means of *curve-fitting*.

One commonly used method of emulation is *Gaussian-process emulation* (GPE). In the astrophysical literature it has been used to fit 