Author(s): Annie Robin
The Universe Model developed for the simulation of the mission is a set of algorithms for computing the positions at any time, and observational properties of objects expected to be observed by the Gaia instruments.
The distributions of these objects and the statistics of observables are based on state-of-the-art descriptions of the characteristics of Gaia sources and on realistic scenarii for the formation, evolution and dynamics of these objects. The complexity of the Universe model increased during the preparation of Gaia, and it is subject to changes after each Gaia data release, to stay in reasonable agreement with the data.
The Universe Model simulates Solar System objects (planets, satellites, asteroids, comets), Galactic objects (stars, nebulae, stellar clusters, diffuse light), extragalactic objects (galaxies resolved in stars, unresolved but extended galaxies, quasars and active galactic nuclei, supernovae). For each of these simulated objects, their full 3D spatial distribution is used, together with their spectral characteristics (to be able to compute photometry and spectroscopy, stable or variable in time), and their motions (for astrometric computations and for spectral corrections). Gravitational lensing for stars and galaxies are also simulated. Interstellar extinction is taken into account based on a 3D model.
In order to simulate the sky content, the sky is subdivided in smaller regions with a Hierarchical Triangular Mesh (HTM, see http://skyserver.org/htm/). The source generation uses a consistent random number generator on sky regions so that exactly the same list of sources is always obtained in a given HTM, if no input parameters have been changed. Each source possesses a unique identifier based on this HTM number, the full format of the identifier being described in gaia_universe_model-source_extended_id. Similarly, all the characteristics of sources are generated with a unique seed, ensuring that for each simulation the same stars have the same characteristics (variability, binarity…).
The relativistic corrections for all types of objects (including aberration and light bending by the Solar System main bodies) use the Gaia Relativity Model (GREM).
Within this data release, only Galactic stellar systems have been generated and therefore only the generation of those will be described in the following sections.
Sources are generated down to magnitude G=21. An approximation of the G magnitude using colour relations is done for this cut, which is different from the final G magnitude computed from the source spectra. For variable sources, the brightest magnitude of the source is used.
The Universe Model outputs (GUMS 20) can be found in the Table gaia_universe_model.