New filamentary remnant radio emission and duty cycle constraints in the radio galaxy NGC 6086^⋆

¹ Dipartimento di Fisica e Astronomia, Università di Bologna, via P. Gobetti 93/2, 40129 Bologna, Italy
e-mail: simone.candini@studio.unibo.it
² INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, via Gobetti 93/3, 40129 Bologna, Italy
³ INAF – Istituto di Radioastronomia, Bologna Via Gobetti 101, 40129 Bologna, Italy
⁴ INAF – Osservatorio Astronomico di Cagliari, Via della Scienza 5, 09047 Selargius (CA), Italy
⁵ Leiden Observatory, Leiden University, PO Box 9513 2300 RA, Leiden, The Netherlands
⁶ Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138, USA
⁷ Universität Hamburg, Hamburger Sternwarte, Gojenbergsweg 112, 21029 Hamburg, Germany

Received: 29 May 2023
Accepted: 21 June 2023

Abstract

Radio galaxies are a subclass of active galactic nuclei (AGN) in which accretion onto the supermassive black hole releases energy into the environment via relativistic jets. The jets are not constantly active throughout the life of the host galaxy and alternate between active and quiescent phases. Remnant radio galaxies are detected during a quiescent phase and define a class of unique sources that can be used to constrain the AGN duty cycle. We present, for the first time, a spatially resolved radio analysis of the radio galaxy associated with the galaxy NGC 6086 down to 144 MHz and constraints on the spectral age of the diffuse emission to investigate the duty cycle and evolution of the source. We used three new low-frequency, high-sensitivity observations; the first was performed with the Low Frequency Array at 144 MHz and the other two with the upgraded Giant Metrewave Radio Telescope at 400 MHz and 675 MHz, respectively. To these, we add two Very Large Array archival observations at higher frequencies (1400 and 4700 MHz). In the new observations in the frequency range 144–675 MHz, we detect a second pair of larger lobes and three regions within the remnant emission with a filamentary morphology. We analysed the spectral index trend in the inner remnant lobes and see systematically steeper values (α_low∼1.1–1.3) at the lower frequencies compared to the gigahertz frequencies (α_high∼0.8–0.9). Steeper spectral indices are found in the newly detected outer lobes (up to α_outer∼2.1), as expected if they trace a previous phase of activity of the AGN. However, the differences between the spectra of the two outer lobes suggest different dynamical evolutions within the intra-group medium during their expansion and/or different magnetic field values. Using a single-injection radiative model and assuming equipartition conditions, we place constraints on the age of the inner and outer lobes and derive the duty cycle of the source. We estimate that the duration of the two active phases was 45 Myr and 18 Myr and the duration of the two inactive phases was 66 Myr and 33 Myr. This results in a total active time of t_on ∼ 39%. The filamentary structures have a steep spectral index (∼1) without any spectral index trend, and only one of them shows a steepening in the spectrum. Their origin is not yet clear, but they may have formed due to the compression of the plasma or due to magnetic field substructures.