Fig. 4
Histogram of optical-VLBI total positional differences for 1297 ICRF2 sources identified in the SDSS DR9 within a search radius of 500 mas. The distribution is consistent with ~57 mas (1σ), assuming Rayleigh distribution. The vertical dashed line indicates the 3σ value of 170 mas. A total of 51 sources have optical-radio offsets larger than 3σ.
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