Celestial reference frames in the Gaia era
Résumé
A working meeting about Gaia and celestial reference frames, funded by the AS-Gaia, took place in Bordeaux in October 2008. Researchers from Paris, Nice and Bordeaux observatories met in order to lay the foundations of future studies and collaborations in this field. The fundamental celestial reference system is materialized by the International Celestial Reference Frame (ICRF), currently based on the VLBI (Very Long Baseline Interferometry) position of 717 extragalactic radio sources with sub-milliarcsecond accuracy. On the basis of about 10 000 Quasi Stellar Objects (QSOs), Gaia will permit to create its own celestial reference frame by 2015-2020, with an unprecedented positional accuracy (ranging from a few tens of microarcseconds at magnitude 15-18 to about 200 microarcseconds at magnitude 20). For consistency between optical and radio positions, it will be important in the future to align these two frames (the Gaia reference frame and the ICRF) with the highest accuracy. This alignment will be important not only for guaranteeing the proper transition if moving from the radio domain to the optical domain, but also for registering the radio and optical images of any celestial target with the highest accuracy. In this paper, we will present the context and the goals of this meeting, review the work carried out in each of the three observatories, and present the outcome of the meeting as well as prospects for the future.