VLBI morphology and astrometric suitability of reference frame sources

The compact extragalactic radio-emitting objects used to define the International Celestial Reference Frame (ICRF) are only imperfect fiducial points in the sky on milliarcsecond scales. Many sources show frequency- and time-dependent extended emission structures with varied morphologies that are understood in the framework of unified theories of active galactic nuclei. Large imaging surveys conducted with Very Long Baseline Interferometry (VLBI) arrays are now available to evaluate the astrometric suitability of most of the ICRF sources. Overall, half of the ICRF sources have excellent or good astrometric suitability. The rest of the sources shows spatially-extended structures which may not be appropriate for defining the celestial reference frame with the highest level of accuracy. Astrometric suitability is found to improve when observing at higher VLBI frequencies