Imploding Remnants: detection bias against AGNs in massive clusters

Abstract

We propose that an observed scarcity of remnant lobed AGNs in dense clusters results from a peculiarity in their dynamics upon the cessation of jet activity: a rapid `implosion' of lobes that, in their active phase, were primarily supported by the momentum flux of the jet. We investigate this behaviour by analysing the asymptotic behaviour of the RAiSE dynamical model and comparing our predictions both to the full model and hydrodynamic simulations. We find that remnant lobes powered by weak jets in massive clusters are unstable to implosion on the order of at most a few Myr. Consequently, remnant AGNs in massive clusters ($M_\text{halo} \sim 10^{14.5}$~M$_\odot$) will be under-counted by a factor of at least five compared to those in poorer groups ($M_\text{halo} \sim 10^{12}$~M$_\odot$). The lack of such remnants in observed populations may lead to a significant underestimate of the AGN feedback provided by low-powered jets, especially given their prevalence towards cluster cores where feedback is most effective. We discuss the influence of a stabilising magnetic field sheath on the nature of the implosion: does the lobe cleanly implode in on itself, or do fluid instabilities turbulently mix the lobe and ambient medium?

Figures (6)

• Figure 1: Schematic of the Turner+2023a dynamical model for the lobe and shocked shell (\ref{['sec:pressure-driven expansion']}), showing the scission of the two lobes immediately upon the cessation of jet activity (\ref{['sec:buoyancy-driven expansion']}).
• Figure 2: The implosion timescale ($t_{\rm crit}/t_\text{on}$) derived using the RAiSE dynamical model for remnant lobes across the jet power--ambient medium $(Q_\text{tot}, M_{\rm halo})$ parameter space. The implosion timescale is shown in red/pink shading for three active ages: 1, 10 and 100 Myr. The asymptotic analysis upper bound for parameters that lead to implosion is shown (blue shaded lines) for comparison assuming plausible values of the slopes of the approximating power laws, $\alpha$ and $\beta$. The red star marks the location in parameter space assessed against a hydrodynamic simulation in \ref{['sec:diffusion and fluid instabilities']}.
• Figure 3: The visible timescale ($t_{\rm vis}/t_\text{on}$) at 150 MHz derived using the RAiSE dynamical model for remnant lobes across the jet power--ambient medium $(Q_\text{tot}, M_{\rm halo})$ parameter space. The visible timescale is shown in blue shading for three active ages: 1, 10 and 100 Myr. The regions of parameter space below the blue line have a synchrotron cooling timescale comparable to (or longer than) the implosion timescale.
• Figure 4: The mean visible timescale (at 150 MHz) of remnant lobes expected across an observed population as a function of cluster mass. The occurrence of each jet power is weighted as $p(Q) \propto Q^{-1.5}$ following Quici+2025. The visible timescale is shown for three active ages: 1, 10 and $100$ Myr in red, orange and blue, respectively. The occurrence of each active age is additionally weighted as $p(t_\text{on}) \propto t_\text{on}^{-1}$Quici+2025 to give a prediction for the overall population (grey dashed line).
• Figure 5: Comparison of the implosion timescale derived using the RAiSE dynamical model either including or excluding a buoyant force along the jet axis. The difference in the implosion timescales ($\Delta t_{\rm crit}/t_\text{on}$; longer timescale with buoyancy) is shown for remnant lobes across the jet power--ambient medium $(Q_\text{tot}, M_{\rm halo})$ parameter space; see \ref{['fig:tcrit']} for complete description. The pixelation in the centre panel results from numerical errors in regions of parameter space that are quasi-stable to implosion.