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

Abstract

Radio galaxies are a subclass of active galactic nuclei in which accretion onto the supermassive black hole releases energy 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 to constrain the AGN duty cycle. We present, a spatially resolved radio analysis of the radio galaxy associated with NGC 6086 and constraints on the spectral age of the diffuse emission to investigate the duty cycle and evolution of the source. We use three new low-frequency, high-sensitivity observations, performed with the Low Frequency Array at 144 MHz and with the upgraded Giant Metrewave Radio Telescope at 400 MHz and 675 MHz. To these, we add two Very Large Array archival observations at 1400 and 4700 MHz. In the new observations, we detect a second pair of larger lobes and three regions with a filamentary morphology. We analyse the spectral index trend in the inner remnant lobes and see systematic steeper values at the lower frequencies compared to the GHz ones. Steeper spectral indices are found in the newly detected outer lobes (up to 2.1), as expected if they trace a previous phase of activity of the AGN. However, the differences between the spectra suggest different dynamical evolution within the intragroup medium during their expansion and/or different magnetic field values. We place constraints on the age of the inner and outer lobes and derive the duty cycle of the source. This results in a total active time of $\sim$39%. The filamentary structures have a steep spectral index ($\sim$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.