Origin of the ring structures in Hercules A

Sub-arcsecond 144 MHz to 7 GHz observations^★

¹ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
e-mail: rtimmerman@strw.leidenuniv.nl
² ASTRON, Netherlands Institute for Radio Astronomy, Oude Hoogeveensedijk 4, Dwingeloo, 7991 PD, The Netherlands
³ National Radio Astronomy Obs, 520 Edgemont Rd., Charlottesville, VA 22903, USA
⁴ National Radio Astronomy Obs, PO Box 0, Socorro, NM 87801, USA
⁵ Centre for Extragalactic Astronomy, Department of Physics, Durham University, Durham DH1 3LE, UK
⁶ Institute for Computational Cosmology, Department of Physics, University of Durham, South Road, Durham DH1 3LE, UK
⁷ Hellenic Open University, School of Science & Technology, Parodos Aristotelous 18, Perivola Patron, Greece
⁸ Department of Physics & Astronomy, University of Manitoba, Winnipeg, MB R3T 2N2, Canada
⁹ Harvard-Smithsonian Center for Astrophysics, 60 Garden St., Cambridge, MA 02138, USA
¹⁰ National Centre for Radio Astrophysics, S. P. Pune University Campus, Post Bag 3, Ganeshkhind Pune 411 007, India
¹¹ Remote Sensing Division, Naval Research Laboratory, Code 7213, 4555 Overlook Ave SW, Washington DC 20375, USA
¹² GEPI & USN, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92190 Meudon, France
¹³ Centre for Radio Astronomy Techniques and Technologies, Department of Physics and Electronics, Rhodes University, Grahamstown 6140, South Africa
¹⁴ Hamburger Sternwarte, University of Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹⁵ Instituto de Astrofísica de Andalucía (IAA, CSIC), Glorieta de las Astronomía, s/n, 18008 Granada, Spain
¹⁶ INAF–Istituto di Radioastronomia, Via Gobetti 101, 40129 Bologna, Italy

Received: 25 March 2021
Accepted: 1 July 2021

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.