Milliarcsecond-resolution Radio-imaging Survey of Blazar Candidates at 4 < z < 5.4

TL;DR

This study uses milliarcsecond-resolution VLBI radio imaging to confirm the blazar nature of high-redshift candidates, revealing that most are blazars with compact cores, thus expanding the known population of early Universe AGN.

Contribution

First high-resolution VLBI survey of z > 4 blazar candidates, confirming their blazar nature and increasing the sample of such objects by approximately 10%.

Findings

11 out of 17 sources show Doppler-boosted emission consistent with blazars.

5 sources lack clear Doppler boosting, possibly due to variability or borderline classification.

2 sources are likely non-blazars.

Abstract

We present a milliarcsecond-resolution radio survey of 17 high-redshift (4 < z < 5.4) blazar candidates observed with the European Very Long Baseline Interferometry (VLBI) Network at 5 GHz. The primary objective of this study was to investigate the nature of these distant active galactic nuclei (AGN) and to confirm their blazar nature. Utilizing the technique of VLBI, we obtained high-resolution radio images of compact core and core-jet structures. To confirm the classification of these sources, we collected multi-band archival data, including total radio flux densities from single-dish and low-resolution interferometric surveys, optical astrometric positions from Gaia, and X-ray data. These diagnostics collectively help distinguish between blazars and misaligned jetted AGN. We were able to measure the core brightness temperatures and found that 11 objects show the Doppler-boosted emission expected from blazars. For five additional sources, we do not see evidence of Doppler boosting even if X-ray data suggest that the source is a blazar. These could be either borderline objects or variability may have affected the classifications, considering that VLBI and X-ray data are not simultaneous. Finally, for the two remaining objects the data suggest a non-blazar classification. Our findings confirm that a significant fraction of these high-redshift radio-loud quasars are blazars and mainly characterized by compact core structures. Overall, the VLBI classifcations are consistent with the X-ray classes. This study further increases the sample of VLBI-imaged radio quasars at z > 4 by ~10%, offering valuable on the population of AGN in the early Universe.