First VLBI detection of Fornax A

TL;DR

This paper reports the first VLBI detection of Fornax A's core, revealing a compact, high-brightness, non-thermal source consistent with an unresolved active galactic nucleus, and investigates extended emission and polarization properties.

Contribution

It provides the first VLBI imaging of Fornax A's core, constraining its size, flux, and polarization, and explores the presence of extended flux and additional sources.

Findings

Detected the compact core of Fornax A at 8.4 GHz.

Core flux density constrained to 47.5-62.3 mJy.

Brightness temperature exceeds 10^11 K, indicating non-thermal emission.

Abstract

Radio galaxies harbouring jetted active galactic nuclei are a frequent target of very-long-baseline interferometry (VLBI) because they play an essential role in exploring how jets form and propagate. Hence, only few have not been detected with VLBI yet; Fornax A is one of the most famous examples. Here we present the first detection of the compact core region of Fornax A with VLBI. At 8.4 GHz the faint core is consistent with an unresolved point source. We constrained its flux density to be $S_0 = 47.5-62.3\,\textrm{mJy}$ and its diameter to be $D^\textrm{min}_0 \leq 70\,\mu\textrm{as}$. The high values of the measured brightness temperature ($T_\textrm{B} \gtrsim 10^{11}\,\textrm{K}$) imply that the observed radiation is of non-thermal origin, likely associated with the synchrotron emission from the active galactic nucleus. We also investigated the possibility of a second radio source being present within the field of view. Adding a second Gaussian component to the geometrical model-fit does not significantly improve the quality of the fit and we, therefore, conclude that our detection corresponds to the compact core of Fornax A. Analysis of the non-trivial closure phases provides evidence for the detection of more extended flux density, on the angular scale of $\sim4000\,\mu\textrm{as}$. Finally, the fractional circular polarisation of the core is consistent with zero, with a conservative upper limit being $m_\textrm{circ} \leq 4\%$.