Introduction

``Galaxies are to astronomy what atoms are to physics''

Allan Sandage

The understanding of the structure, formation and evolution of galaxies is certainly one of the most important challenges for contemporary astronomy, not only because galaxies are important in their own right, but also because their study can provide clues to new laws of physics.

In recent years, extragalactic astronomy has experienced a great progress both from the observational and theoretical point of view, thanks to developments in both instrumentation and numerical techniques. On the observational side, this field of astronomical research has traditionally been dominated by large-aperture ground-based telescopes, capable of peering deeply into the sky. With the advent of the Hubble Space Telescope, however, and particularly thanks to the superb, large-field-of-view images provided by WF/PC and WFPC2, the dramatic scientific case for extragalactic studies at high spatial resolution, presently achievable from above the Earth's atmosphere only, has been fully demonstrated, to the extent that the next planned large-aperture space observatory, the Next Generation Space Telescope, has as its main scientific goal the investigation of the formation and evolution of galaxies.

In this historical context, following on the huge success of Hipparcos, the first dedicated astrometric satellite, some three years ago ESA has started, in collaboraton with the European astronomical community, the feasibility studies for GAIA, a revolutionary astrometric satellite mission. According to its present design, established after extensive industrial and scientific studies, the GAIA satellite will perform astrometric and photometric measurements of unprecedented accuracy over a magnitude-limited sample of about a billion stars brighter than $V\simeq20$. For more than a hundred million stars, the satellite will also perform radial velocity measurements so as to identify the position of stars in the six-dimensional phase-space, in the first stereoscopic survey of our Galaxy. The GAIA mission is now a short-listed candidate for selection by the European Space Agency as Cornerstone 5 of its Horizons 2000 Scientific Programme. Selection is expected in September 2000 and, if selected, GAIA could be launched around 2009.

In the framework of the feasibility studies for the GAIA mission, the opportunity of performing galaxy astrometric and photometric observations in parallel with star observations was suggested in early 1998 by Prof. Erik Høg from the Astronomical Observatory of the University of Copenhagen. This thesis, carried out in part at the Astronomical Observatory of the University of Copenhagen and in part at the Asiago Astrophysical Observatory of the University of Padova, is the result of a study intended to demonstrate the feasibility and scientific potential of such observations. Preliminary results were presented in three reports ([Vaccari and Høg 1999a], [Vaccari, Høg and Makarov 1999] and [Vaccari and Høg 1999b]) to the Science Advisory Group established by ESA for the GAIA mission, which has now included the observation of galaxies in the mission baseline design under the name of GAIA Galaxy Survey.

The GAIA Galaxy Survey is basically a nearly all-sky, high-spatial-resolution, multi-color and multi-epoch astrometric and photometric survey of the central regions of galaxies brighter than $I\simeq17$. More than 3 million galaxies would be observed in at least 4 colors with a spatial resolution better than 0.4 arcsec. Such observations would yield a large, high-quality and uniform dataset that could be used for statistical studies of spatial distribution and core photometric structure of bright galaxies down to low Galactic latitudes. The galaxy spatial distribution on the sky will be used to probe the large-scale structure of the Local Universe, whereas high-spatial-resolution surface photometry of the galaxy innermost regions will shed light onto the wealth of structures that appear to be present at the center of most galaxies. In the foreseeable future, an observation campaign of this kind could not be carried out by other telescopes, either from the ground, due to seeing and related difficulties in star-galaxy discrimination, or from space, due to the premium on the observing time of space observatories. Galaxy observations could instead easily be obtained by GAIA with only a small effort in terms of mission design, data transmission and analysis. Besides providing the astronomers with an unprecedented wealth of fundamental information on our Galaxy, the GAIA mission could thus also yield a significant contribution to the study of external galaxies.