3.3 The GAIA Galaxy Survey

Since the time of the Herschels, surveys of bright galaxies have provided the foundations upon which much of observational cosmology rests. Traditionally, these have been carried out on large-scale photographic plates, and only with the recent Sloan Digital Sky Survey CCDs have been successfully employed to image a significant portion of the northern sky. Owing to the revolutionary nature of the GAIA mission, the GAIA Galaxy Survey has little in common with previous galaxy survey projects, its main advantages being the nearly all-sky coverage, a well-defined selection function and a very high angular resolution. The overall measurement capabilities expected from the GAIA Galaxy Survey are described in the following Chapters. For the time being, and for a general discussion of its scientific objectives, it suffices to say that multi-color, multi-epoch photometry at a resolution better than 0.4 arcsec for some 3 million galaxies will be obtained.

Growth of structure in the Universe is believed to proceed from small-amplitude perturbations at very early times. Their growth from the radiation-dominated era to the present has been extensively studied, particularly in the context of the popular hierarchical clustering scenario. Many aspects of this picture are fairly well-established. Others are subject of active definition through redshift and imaging surveys of galaxies, and the microwave background experiments. There are several aspects of this research which require very wide area imaging surveys with high angular resolution to provide high-reliability catalogues of galaxies and quasars extending to low Galactic latitudes. Here GAIA will contribute uniquely, by detecting and providing multi-color surface photometry at high angular resolution for all sufficiently high surface brightness galaxies. This provides a valuable and unique dataset at two levels: for study of the large-scale structure of the Local Universe probed through the spatial distribution of galaxies and for statistical studies of the photometric structure of their central regions.