First VLBI detection of Fornax A

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 $S_0=47.5-62.3\text{mJy}$ and its diameter to be $D_0^{\text{min}}\le 70\hat{I}¼\text{as}$. The high values of the measured brightness temperature ($T_{\text{B}}\gtrsim 10^{11}\text{K}$) 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 $\sim 4000\hat{I}¼\text{as}$. Finally, the fractional circular polarisation of the core is consistent with zero, with a conservative upper limit being $m_{\text{circ}}\le 4\%$.

4 pages, 2 figures, accepted for publication in A&A