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The ICRF, currently based on the position of 717 extragalactic radio sourcesobserved by VLBI, is the fundamental celestial reference frame adopted by theIAU in 1997. Within the next 10 years, the European space astrometry missionGaia, to be launched by 2011, will permit determination of the extragalacticreference frame directly in the visible for the first time. Aligning these twoframes with the highest accuracy will therefore be very important in the futurefor ensuring consistency between the measured radio and optical positions. Thispaper is aimed at evaluating the current astrometric suitability of theindividual ICRF radio sources which are considered appropriate for thealignment with the future Gaia frame. To this purpose, we cross-identified theICRF and the optical catalog V\'eron-Cetty and V\'eron (2006), to identify theoptically-bright ICRF sources that will be positioned with the highest accuracywith Gaia. Then we investigated the astrometric suitability of these sources byexamining their VLBI brightness distribution. We identified 243 candidate ICRFsources for the alignment with the Gaia frame (with an optical counterpartbrighter than the apparent magnitude 18), but only 70 of these (10% of the ICRFsources) are found to have the necessary high astrometric quality (i.e. abrightness distribution that is compact enough) for this link. Additionally, itwas found that the QSOs that will have the most accurate positions in the Gaiaframe tend to have less-accurate VLBI positions, most probably because of theirphysical structures. Altogether, this indicates that identifying otherhigh-quality VLBI radio sources suitable for the alignment with the future Gaiaframe is mandatory.

Astrometric suitability of optically-bright ICRF sources for the alignment with the future Gaia celestial reference frame