The flickering nuclear activity of Fornax A

TL;DR

This study uses new radio observations to reveal that Fornax A's lobes have a double-shell structure, with evidence of multiple recent episodes of nuclear activity and a complex merger history influencing its flickering activity.

Contribution

First detailed spectral analysis combining MeerKAT, SRT, and archival data showing multiple nuclear activity episodes in Fornax A.

Findings

Fornax A's lobes have a double-shell morphology with dense filaments.

Radio spectrum modeling indicates episodic particle injection over the past 24 Myr.

The galaxy's recent merger history likely causes its rapid, recurrent nuclear activity.

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 micro-Jy 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 approximately 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 in which Fornax A lives 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.