On significance of VLBI/Gaia position offsets

TL;DR

This study compares Gaia optical positions with VLBI radio positions for over 11,000 sources, revealing a significant subset with genuine positional offsets indicating different emission regions.

Contribution

It provides the first large-scale statistical analysis of radio-optical position offsets, highlighting their significance and potential astrophysical implications.

Findings

6% of matched sources have significant radio-optical offsets

Median VLBI position uncertainties are four times smaller than optical counterparts

Distribution of normalized offsets deviates from expected Rayleigh distribution

Abstract

We have cross matched the Gaia Data Release 1 secondary dataset that contains positions of 1.14 billion objects against the most complete to date catalogue of VLBI positions of 11.4 thousand sources, almost exclusively active galactic nuclei. We found 6,064 matches, i.e. 53% radio objects. The median uncertainty of VLBI positions is a factor of 4 smaller than the median uncertainties of their optical counterparts. Our analysis shows that the distribution of normalized arc lengths significantly deviates from Rayleigh shape with an excess of objects with small normalized arc lengths and with a number of outliers. We found that 6% matches have radio optical offsets significant at 99% confidence level. Therefore, we conclude there exists a population of objects with genuine offsets between centroids of radio and optical emission.