3. Scientific Case for Galaxy Observations with GAIA

As we saw in the previous Chapter, the GAIA mission was essentially conceived with the purpose of precisely measuring star positions in order to disentangle the stars' parallactic and proper motions. Consequently. the mission design resulting from the preliminary studies, and whose main features are a continuous, relatively quick rotation of the satellite (i.e. short single-scan exposure times) and rectangular telescope apertures (i.e. elliptical Airy Disks), is certainly more suitable for the accurate centroiding of point-like sources than for two-dimensional morphological studies of diffuse sources.

Still, as soon as the mission design was firmly established, it was realized that the scientific case for two-dimensional observations of high-surface brightness regions with GAIA was potentially dramatic. Although such an idea came up relatively late in the course of the misssion feasibility studies, it was soon integrated in the mission baseline design under the name of GAIA Galaxy Survey.

Accurate positional measurements and high-resolution, multi-color photometric observations of the inner regions of about 3 million galaxies are unique datasets GAIA could provide with only a minimum effort in terms of mission design and telemetry. Owing to the late inclusion of galaxy observations in the mission design, it has not yet been possible to consider in detail the wide-ranging scientific objectives that could thus be addressed, and the objectives that are here briefly mentioned are therefore to be considered a minimum.