Plans for an accurate alignment of the VLBI frame and the future Gaia frame

The European space astrometry mission Gaia will construct a dense optical QSO-based celestial reference frame. For consistency between optical and radio positions, it will be fundamental to align the Gaia and VLBI frames with the highest accuracy. A proper alignment is also important in the framework of astrophysics, for example to probe properly the AGN jets properties and the physics of these objects. The VLBI-Gaia frame alignment requires quasars that are bright at optical wavelength, that have a compact radio core, and that do not exhibit complex structures. In this paper, we draw prospects for this alignment, based on the ICRF2 catalogue and an ongoing dedicated VLBI project designed to observe additional weaker extragalactic radio sources for this purpose. The list of suitable sources will have to be monitored to check the relevance of the sources for the alignment, especially in terms of position stability and structures. Accordingly, we present the observations we envision in the framework of the IVS and other VLBI networks, before and during the Gaia mission.