Finding AGN with wide-field VLBI observations

TL;DR

This study demonstrates an efficient VLBI method to identify AGN in deep fields, detecting previously unknown sources and revealing limitations of X-ray data for AGN identification.

Contribution

The paper introduces an extended correlator technique enabling high-sensitivity VLBI imaging of many sources in a short time, improving AGN detection capabilities.

Findings

Detected 20 sources out of 96, including 8 new AGN candidates.

X-ray data alone cannot reliably identify all AGN.

Type 1 QSOs are always detected in VLBI, unlike radio-loud QSOs.

Abstract

VLBI observations are a reliable method to identify AGN, since they require high brightness temperatures for a detection to be made. However, because of the tiny fields of view it is unpractical to carry out VLBI observations of many sources using conventional methods. We used an extension of the DiFX software correlator to image with high sensitivity 96 sources in the Chandra Deep Field South, using only 9h of observing time with the VLBA. We detected 20 sources, 8 of which had not been identified as AGN at any other wavelength, despite the comprehensive coverage of this field. The lack of X-ray counterparts to 1/3 of the VLBI-detected sources, despite the sensitivity of co-located X-ray data, demonstrates that X-ray observations cannot be solely relied upon when searching for AGN activity. Surprisingly, we find that sources classified as type 1 QSOs using X-ray data are always detected, in contrast to the 10% radio-loud objects which are found in optically-selected QSOs. We present the continuation of this project with the goal to image 1450 sources in the Lockman Hole/XMM region.