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Link between the VLBI and Gaia Reference Frames

The link between the International Celestial Reference Frame at radio wavelength and the forthcoming Gaia optical reference frame is a mandatory task after the completion of the Gaia mission. Starting from the provisional reference frame in which Gaia astrometric solutions were obtained, we discuss the ways to correct the residual rotation and acceleration effects and investigate three potential options for linking the two frames realized by extragalactic sources. We have estimated the accuracy for the frame alignment assuming different astrometric models of quasar proper motions observed by very long baseline interferometry (VLBI). Using about 500,000 high-precision proper motions of extragalactic sources, the residual rotation of the Gaia reference frame is evaluated under 1 μas yr−1. In view of its favorable properties, Gaia should be given priority to be considered as the future fundamental reference frame that is consistent with the principle of the International Celestial Reference System. The VLBI reference frame can be linked to Gaia based on thousands of common quasars with an accuracy of 10 μas for each axis.