Alignment and distortions of the Gaia celestial reference frame and the three components of ICRF3

The International Celestial Reference Frame, version 3, provides the best available realization of an inertial, non-rotating, system of celestial coordinates. This presentation describes in-depth analysis of the three components of ICRF3 (observed in the S/X, K, and X/Ka wave bands) relative to the optical counterparts in the Gaia EDR3 catalog. The three vector fields of position differences Gaia - ICRF3 are fitted with uniform sets of 48 vector spherical harmonics up to degree 4. Significant differences are found between these decompositions. The smallest Euler rotation angles, represented by the magnetic harmonic terms of first degree, are found for the S/X catalog as expected, where the largest estimate has a S/N = 1.3. The only statistically significant harmonic with a S/N > 3 is EVSH_020, which is a zonal pattern with vectors in both hemispheres directed toward the equator. The fit for the K catalog also includes only one term with a S/N > 3, which is EVSH_211. The X/Ka band catalog, on the other hand, has a much greater and radically different distortion pattern dominated by several low-degree harmonics, the most significant one with a S/N > 9 being the zonal EVSH_010, which is a streaming field from the south pole to the north pole. This part of ICRF3 cannot be used as intended without appropriate corrections. The overall rates of significant radio-optical position offsets also widely differ between the components. Above the 3 sigma level of confidence, the rates are 25%, 19%, and 35% for the S/X, K, and X/Ka bands, respectively. The detected distortions are likely to originate in the ICRF3 parts rather than in the optical Gaia CRF.