Studying the link between radio galaxies and AGN fuelling with relativistic hydrodynamic simulations of flickering jets

ABSTRACT We present two- and three-dimensional hydrodynamic simulations of ∼kpc-scale AGN jets with mean jet powers in the range 1–7 × 1045 erg s−1, in which the jet power varies (through variation of the Lorentz factor) according to a flicker or pink noise power spectrum. We find the morphology and dynamics of the jet–cocoon system depends on the amplitude of the variability with a clear correspondence between the shape of the cocoon and the historical activity. The jet advances quickly during high-power states, whereas quiescent periods instead produce passive periods of inflation resembling Sedov–Taylor blast waves. Periods of high activity preferentially produce hotspots and create stronger backflow as they maximize the pressure gradient between the jet head and cocoon. The variability can also lead to propagating internal shock structures along the jet. Our work suggests that variability and flickering in the jet power has important implications, which we discuss, for observations of radio galaxies, ultrahigh energy cosmic ray acceleration and jet power to luminosity correlations. We explore the link between morphology and fuelling, and suggest that chaotic cold accretion should introduce a relatively small scatter in radio luminosity (∼0.2 dex) and modest imprints on morphology; sources such as Hercules A and Fornax A, which show evidence for more dramatic variability, may therefore require redder power spectra, or be triggered by mergers or other discrete events. We suggest ways to search for jet flickering observationally and propose that radio galaxies may be an important diagnostic of Myr time-scale AGN fuelling, due to their ‘long-term memory’.