Letter to the Editor

First very long baseline interferometry detection of Fornax A

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: gfparaschos@mpifr-bonn.mpg.de
² Institute of Physics, Silesian University in Opava, Bezručovo nám. 13, 746 01 Opava, Czech Republic
³ Department of Physics, National and Kapodistrian University of Athens, Panepistimiopolis, 15783 Zografos, Greece
⁴ Institut für Theoretische Physik, Goethe Universität Frankfurt, Max-von-Laue-Str.1, 60438 Frankfurt am Main, Germany
⁵ Julius Maximilians University Würzburg, Faculty of Physics and Astronomy, Institute for Theoretical Physics and Astrophysics, Chair of Astronomy, Emil-Fischer-Str. 31, 97074 Würzburg, Germany
⁶ NASA HQ, 300 E St SW, Washington, DC 20546-0002, USA
⁷ CSIRO Space and Astronomy, PO Box 76, Epping, NSW 1710, Australia
⁸ School of Natural Sciences, University of Tasmania, Private Bag 37, Hobart, Tasmania 7001, Australia
⁹ Hartebeesthoek Radio Astronomy Observatory, PO Box 443 1740 Krugersdorp, South Africa
¹⁰ Space Operations New Zealand Ltd, 62 Deveron Street, Invercargill 984, New Zealand

Received: 2 May 2024
Accepted: 4 June 2024

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 our exploration of how jets form and propagate. Hence, only a few have not yet been detected with VLBI; Fornax A was 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 S₀ = 47.5 − 62.3 mJy and its diameter to be D₀^min ≤ 70 μas. The high values of the measured brightness temperature (T_B ≳ 10¹¹ 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 a more extended flux density, on the angular scale of ∼4000 μas. Finally, the fractional circular polarisation of the core is consistent with zero, with a conservative upper limit being m_circ ≤ 4%.