Galaxia: A Code to Generate a Synthetic Survey of the Milky Way

We present here a fast code for creating a synthetic survey of the Milky Way. Given one or more color–magnitude bounds, a survey size, and geometry, the code returns a catalog of stars in accordance with a given model of the Milky Way. The model can be specified by a set of density distributions or as an N -body realization. We provide fast and efficient algorithms for sampling both types of models. As compared to earlier sampling schemes which generate stars at specified locations along a line of sight, our scheme can generate a continuous and smooth distribution of stars over any given volume. The code is quite general and flexible and can accept input in the form of a star formation rate, age–metallicity relation, age–velocity-dispersion relation, and analytic density distribution functions. Theoretical isochrones are then used to generate a catalog of stars, and support is available for a wide range of photometric bands. As a concrete example, we implement the Besançon Milky Way model for the disk. For the stellar halo we employ the simulated stellar halo N -body models of Bullock & Johnston. In order to sample N -body models, we present a scheme that disperses the stars spawned by an N -body particle, in such a way that the phase-space density of the spawned stars is consistent with that of the N -body particles. The code is ideally suited to generating synthetic data sets that mimic near future wide area surveys such as GAIA , LSST, and HERMES. As an application we study the prospect of identifying structures in the stellar halo with a simulated GAIA survey. We plan to make the code publicly available.