Extragalactic radio source selection for use in directly linking optical astrometric observations to the radio reference frame

Future space-based optical astrometric satellite missions present the possibility of directly linking the radio and optical reference frames at the microarcsecond level. We have evaluated the current database of radio observations of the extragalactic objects that make up the International Celestial Reference Frame (ICRF) to determine the optimum candidates in terms of their radio properties for use as radiooptical frame tie sources. We use up-to-date radio astrometric and ancillary data to evaluate the sources in terms of their suitability for use by the Space Interferometry Mission (SIM), but the results apply equally well to any optical astrometric satellite or optical ground-based observations with sufficient brightness sensitivity to measure precise astrometric positions of extragalactic objects directly. Currently, the ICRF limits the accuracy of any radio-optical frame tie based on future optical astrometric satellite observations if the projected accuracies for these missions are realized. We have attempted to construct a suitable set of frame tie sources but found a significant deficit of candidate sources in the southern hemisphere. If the SIM optical frame is to be successfully tied to the ICRF, additional radio observations are necessary to overcome the limitations of the radio frame.