The flickering nuclear activity of Fornax A^⋆

¹ INAF – Osservatorio Astronomico di Cagliari, via della Scienza 5, 09047 Selargius, CA, Italy
e-mail: filippo.maccagni@inaf.it
² Institute of Astronomy, Graduate School of Science, The University of Tokyo, 2-21-1 Osawa, Mitaka, Tokyo 181-0015, Japan
³ South African Radio Astronomy Observatory, Black River Park, 2 Fir Street, Observatory, Cape Town 7925, South Africa
⁴ Department of Physics and Electronics, Rhodes University, PO Box 94, Makhanda 6140, South Africa
⁵ Argelander-Institut für Astronomie, Auf dem Hügel 71, 53121 Bonn, Germany
⁶ Ruhr-University Bochum, Faculty of Physics and Astronomy, Astronomical Institute, 44780 Bochum, Germany
⁷ School of Physics, University of the Witwatersrand, Private Bag 3, 2050 Johannesburg, South Africa

Received: 7 October 2019
Accepted: 20 November 2019

Abstract

We present new observations of Fornax A taken at ∼1 GHz with the MeerKAT telescope and at ∼6 GHz with the Sardinia Radio Telescope (SRT). The sensitive (noise ∼16 μJy beam⁻¹), high-resolution (≲10″) MeerKAT images show that the lobes of Fornax A have a double-shell morphology, where dense filaments are embedded in a diffuse and extended cocoon. We study the spectral properties of these components by combining the MeerKAT and SRT observations with archival data between 84 MHz and 217 GHz. For the first time, we show that multiple episodes of nuclear activity must have formed the extended radio lobes. The modelling of the radio spectrum suggests that the last episode of injection of relativistic particles into the lobes started ∼24 Myr ago and stopped 12 Myr ago. More recently (∼3 Myr ago), a less powerful and short (≲1 Myr) phase of nuclear activity generated the central jets. Currently, the core may be in a new active phase. It appears that Fornax A is rapidly flickering. The dense environment around Fornax A has lead to a complex recent merger history for this galaxy, including mergers spanning a range of gas contents and mass ratios, as shown by the analysis of the galaxy’s stellar- and cold-gas phases. This complex recent history may be the cause of the rapid, recurrent nuclear activity of Fornax A.