Comparison of multifrequency positions of extragalactic sources from global geodetic VLBI monitoring program and Gaia EDR3

TL;DR

This study compares source positions across multiple radio frequencies and Gaia optical data, revealing frequency-dependent positional offsets and their relation to source properties, aiding celestial reference frame alignment.

Contribution

It extends previous analyses by including K and X/Ka band data, providing new insights into frequency dependence and physical properties affecting source position offsets.

Findings

Optical-to-radio offsets are less than 0.5 mas across frequencies.

Offsets are smaller at K band for sources with extended structures.

Offset correlates with source structure index and brightness.

Abstract

We extend previous works by considering two additional radio frequencies (K band and X/Ka band) with the aim to study the frequency dependence of the source positions and its potential connection with the physical properties of the underlying AGN. We compared the absolute source positions measured at four different wavelengths, that is, the optical position from the Gaia Early Data Release 3 (EDR3) and the radio positions at the dual S/X, X/Ka combinations and at K band, as available from the third realization of the International Celestial Reference Frame (ICRF3), for 512 common sources. We first aligned the three ICRF3 individual catalogs onto the Gaia EDR3 frame and compare the optical-to-radio offsets before and after the alignment. Then we studied the correlation of optical-to-radio offsets with the observing (radio) frequency, source morphology, magnitude, redshift, and source type. The deviation among optical-to-radio offsets determined in the different radio bands is less than 0.5 mas, but there is statistical evidence that the optical-to-radio offset is smaller at K band compared to S/X band for sources showing extended structures. The optical-to-radio offset was found to statistically correlate with the structure index. Large optical-to-radio offsets appear to favor faint sources but are well explained by positional uncertainty, which is also larger for these sources. We did not detect any statistically significant correlation between the optical-to-radio offset and the redshift. The radio source structure might also be a major cause for the radio-to-optical offset. For the alignment of with the Gaia celestial reference frame, the S/X band frame remains the preferred choice at present.