A Pattern of Noncoincidence between Radio and Optical Positions of International Celestial Reference Frame Sources

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Abstract
Many of the International Celestial Reference Frame (ICRF) sources are not pointlike, as shown by the 2 GHz and 8 GHz radio maps. The size scale reaches up to a few tens of milliarcseconds for extended sources. Also, although the optical images are not resolved, the centers of emission are not necessarily coincident with the radio centroids. Here we search for indications of such noncoincidence. We divide the sources into two sets, extended and compact, according to the radio structure index given in the ICRF extension. The optical positions are from recent determinations, to obtain the highest precision and evenness of accuracy. The ICRF radio positions are of milliarcsecond precision or better. The average of the absolute values of the differences between the lengths of the optical and radio arcs joining pairs of sources taken within each of the sets is found to be about 7.9 mas larger for the extended sources than for the compact sources. This is interpreted as evidence of noncoincidence between the radio and optical centers, at least for the extended sources. Additional checks made with larger, different sets using the optical source positions from the USNO-A2.0 catalog support this conclusion.