Origin of the ring structures in Hercules A -- Sub-arcsecond 144 MHz to 7 GHz observations

TL;DR

This study uses high-resolution radio observations across multiple frequencies to investigate the origin of ring structures in Hercules A, supporting an inner-lobe formation model linked to past jet activity.

Contribution

First sub-arcsecond low-frequency images of Hercules A combined with VLA data, providing new insights into the spectral properties and origin of the ring structures.

Findings

Spectral curvature increases with distance from the nucleus.

Spectral shapes are consistent with synchrotron aging.

Supports an inner-lobe formation model for the rings.

Abstract

The prominent radio source Hercules A features complex structures in its radio lobes. Although it is one of the most comprehensively studied sources in the radio sky, the origin of the ring structures in the Hercules A radio lobes remains an open question. We present the first sub-arcsecond angular resolution images at low frequencies (<300 MHz) of Hercules A, made with the International LOFAR Telescope. With the addition of data from the Karl G. Jansky Very Large Array, we mapped the structure of the lobes from 144 MHz to 7 GHz. We explore the origin of the rings within the lobes of Hercules A, and test whether their properties are best described by a shock model, where shock waves are produced by the jet propagating in the radio lobe, or by an inner-lobe model, where the rings are formed by decelerated jetted plasma. From spectral index mapping our large frequency coverage reveals that the curvature of the different ring spectra increases with distance away from the central active galactic nucleus. We demonstrate that the spectral shape of the rings is consistent with synchrotron aging, which speaks in favor of an inner-lobe model where the rings are formed from the deposition of material from past periods of intermittent core activity.