Towards ICRF3:preparing the VLBI frame for future synergy with the Gaia frame

International audienceThe European space astrometric mission Gaia to be launched in 2013 will produce a QSO - based celestial reference frame with unprecedented position accuracy and sky density. By the end of the decade, two highly - accurate reference frames will thus cohabit, the International Celestial Reference Frame (ICRF) derived from Very Long Baseline Interferometry (VLBI) data and the Gaia optical frame, both with individual source position accuracies below 100 microarcseconds. For consistency be tween optical and radio positions, it will be fundamental to align the two frames with the highest possible accuracy. This is important not only for continuity of celestial frames but also to exploit at best their synergies for astrophysics. The latter includes probing the Active Galactic Nuclei (AGN) jets properties and the physics of these objects by comparing the spatial location of the optical and radio emission re gions. The alignment between the VLBI and Gaia frames requires a large number of sources common to the two frames, i.e. radio - loud QSOs with position accurately known from both VLBI and Gaia. This implies that the sources must be brighter than magnitude 18 (so that their Gaia positions may be derived with the highest accuracy) and have compact VLBI structure on milliarcsecond scales (for highly - accurate VLBI positions). In this paper, we review the current source potential for this alignment based on the ICRF2 and an ongoing dedicated VLBI project aimed at finding additional weaker extragalactic radio sources for this purpose. We also stress that these sources must be monitored during the mission (especially their VLBI position stability and structure) in order to control their relevance for the alignment, and present the observations we envision to this end in the framework of the IVS and other VLBI networks