CosmoDRAGoN II: Remnant Radio Galaxies in Group and Cluster Environments

TL;DR

This study investigates the active-to-remnant transition of radio galaxies using the CosmoDRAGoN II suite of large-scale 3D hydrodynamic simulations in realistic cluster and group environments. By terminating jet inflows in fifteen progenitor models spanning low and high powers, relativistic and sub-relativistic speeds, and two environmental settings, the work reveals how environment and jet power shape lobe dynamics, bow-shock evolution, remaining overpressure, and mixing. The results show that low-power remnants can stay overpressured longer in low-density environments, delaying buoyant rise, while high-power remnants maintain strong shocks that heat the ambient medium well into the remnant phase; analytic models capturing expansion history align with simulations for certain regimes but fail for jet-dominated cases. Overall, the paper provides a nuanced view of remnant evolution, demonstrates the value of coupling realistic environments with hydrodynamic jets, and highlights remnant feedback pathways relevant for galaxy cluster thermodynamics and AGN duty cycles.

Abstract

Radio galaxy remnants are a rare subset of the radio-loud active galactic nuclei (RLAGN) population, representing the quiescent phase in the RLAGN lifecycle. Despite their observed scarcity, they offer valuable insights into the AGN duty cycle and feedback processes. Due to the mega-year timescales over which the RLAGN lifecycle takes place, it is impossible to observe the active to remnant transition in real-time. Numerical simulations offer a solution to follow the long-term evolution of RLAGN plasma. In this work, we present the largest suite (to date) of three-dimensional, hydrodynamic simulations studying the dynamic evolution of the active-to-remnant transition and explore the mechanisms driving cocoon evolution, comparing the results to the expectations of analytic modelling. Our results show key differences between active and remnant sources in both cluster environments and in lower-density group environments. We find that sources in low-density environments can remain overpressured well into the remnant phase. This significantly increases the time for the remnant lobe to transition to a buoyant regime. We compare our results with analytic expectations, showing that the long-term evolution of radio remnants can be well captured for remnants whose expansion is largely pressure-driven if the transition to a coasting phase is assumed to be gradual. We find that remnants of low-powered progenitors can continue to be momentum-driven for about 10 Myr after the jets switch-off. Finally, we consider how the properties of the progenitor influence the mixing of the remnant lobe and confirm the expectation that the remnants of high-powered sources have long-lasting shocks that can continue to heat the surrounding medium.

Figures (13)

• Figure 1: A schematic of the AGN lifecycle from an active source (left), to a remnant source (middle) to restarted sources (right). If the time for the remnant lobes to fade below the detectable limit is longer than the time for the nuclear activity to reignite, then remnant lobes may be seen together with a newly restarted, young radio source (bottom right). Else, a young radio source may be seen with no indication of past activity (top right).
• Figure 2: Environment density (left) and pressure (right) profiles are shown for the cosmological cluster (orange) and group (green) environments. Respectively, the dashed, dot-dashed and dotted lines show the radial evolution along the z-, x- and y- axes. The solid lines indicate the symmetric, radially averaged fit to the cosmological environments. For comparison, the range of environment profiles considered by the related work of English_2019 is shown by the shaded region.
• Figure 3: Mid-plane slices in the X-Z plane of the logarithmic density distributions for all models at the last active output. The total source age is displayed on each panel. From left to right, the columns show 20 kpc, 60 kpc, and 180 kpc switch-off simulations. The spatial scales have been adjusted across the three columns such that each simulation can be seen clearly. We have grouped the simulations such that low-power sources are shown in the top two rows and high-power sources in the bottom three rows. Group simulations are shown in the panel immediately above the equivalent cluster simulation. The blue contours outline where the passive tracer is above the threshold of $10^{-4}$.
• Figure 4: Mid-plane slices for all models 50 Myr after the remnant phase started. The layout of this figure is analogous to Fig. \ref{['fig:actives']}.
• Figure 5: The time evolution of the total source length measured as the maximum extent (along $z$) of the tracer material above $10^{-4}$. The left-hand panel shows all models with low injected jet kinetic powers ($Q_{\rm{j}} = 10^{36}$ W). Models with high injected jet kinetic powers ($Q_{\rm{j}} = 10^{38}$ W) are shown on the right. The active and remnant phases are shown with thick and thin lines, respectively. The cross-marks indicate $2\times t_{\rm{on}}$. The inset figure shows a zoomed-in view of the 60kpc switch-off low-powered simulations from 0 to 50 Myr showing how these small sources continue to grow as if they were active for some time. The small arrows show the time delay between the point where the jet flow stops and the point where the lobe length evolution begins to slow down.