Letter to the Editor

Milliarcsecond-scale radio structure of the most distant BL Lac object candidate at redshift 6.57

¹ Konkoly Observatory, HUN-REN Research Centre for Astronomy and Earth Sciences, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
e-mail: frey.sandor@csfk.org
² CSFK, MTA Centre of Excellence, Konkoly Thege Miklós út 15-17, 1121 Budapest, Hungary
³ Institute of Physics and Astronomy, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
⁴ Shanghai Astronomical Observatory, Chinese Academy of Sciences, 80 Nandan Road, Shanghai 200030, PR China
⁵ Key Laboratory of Radio Astronomy and Technology, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100101, PR China
⁶ Department of Astronomy, Institute of Physics and Astronomy, ELTE Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
⁷ HUN-REN–ELTE Extragalactic Astrophysics Research Group, Eötvös Loránd University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
⁸ Joint Institute for VLBI ERIC, Oude Hoogeveensedijk 4, 7991 PD Dwingeloo, The Netherlands
⁹ Faculty of Aerospace Engineering, Delft University of Technology, Kluyverweg 1, 2629 HS Delft, The Netherlands
¹⁰ Graduate Institute of Astronomy, National Central University, Taoyuan City 32001, Taiwan

Received: 14 November 2023
Accepted: 25 December 2023

Abstract

Context. The existence of accreting supermassive black holes of up to billions of solar masses at early cosmological epochs (in the context of this work, redshifts z ≳ 6) requires very fast growth rates that are challenging to explain. The presence of a relativistic jet can be a direct indication of activity and accretion status in active galactic nuclei (AGN), constraining the radiative properties of these extreme objects. However, known jetted AGN beyond z ∼ 6 are still very rare.

Aims. The radio-emitting AGN J2331+1129 has recently been claimed as a candidate BL Lac object at redshift z = 6.57 based on its synchrotron-dominated emission spectrum and a lack of ultraviolet or optical emission lines. It is a promising candidate for the highest-redshift blazar known to date. The aim of the observations described here is to support or refute the blazar classification of this peculiar source.

Methods. We performed high-resolution radio interferometric imaging observations of J2331+1129 using the Very Long Baseline Array at 1.6 and 4.9 GHz in February 2022.

Results. The images reveal a compact but slightly resolved, flat-spectrum core feature at both frequencies, indicating that the total radio emission is produced by a compact jet and originates from within a central region of ∼10 pc in diameter. While these details are consistent with the radio properties of a BL Lac object, the inferred brightness temperatures are at least an order of magnitude lower than expected for a Doppler-boosted radio jet, which casts doubt on the high-redshift BL Lac identification.