Cold gas formation triggered by active galactic nuclei jet feedback in galaxy cluster cores

TL;DR

The paper investigates how AGN jet feedback can trigger in situ condensation of cold gas in cool-core galaxy clusters. Using high-resolution Arepo simulations with non-equilibrium chemistry (Grackle) and a detailed SMBH jet feedback model, the authors show that lateral jet expansion in the hot ICM creates compression zones where gas cools rapidly, forming cold clumps on ~30 Myr when the hot gas turbulent Mach number is around 0.3. This positive feedback coexists with global jet-driven heating, reproducing Perseus-like X-ray profiles and yielding realistic distributions of warm and cold gas that match a range of observations. The results provide a physically motivated mechanism linking jet activity, turbulence, and multi-phase gas formation, and offer predictive diagnostics for future X-ray spectroscopy (e.g., XRISM) and high-resolution radio observations to interpret cool-core nebulae in galaxy clusters.

Abstract

Extended warm and cold gas nebulae, with complex morphologies and kinematics, have been observed in the centres of cool-core galaxy clusters. Their origin within the hot intracluster medium (ICM) is still puzzling, and among many mechanisms, positive feedback from the central active galactic nucleus (AGN) has been proposed. In this work, we performed a suite of very high-resolution hydrodynamic simulations of a Perseus-like cool-core galaxy cluster subject to self-regulated AGN jet feedback, which leads to realistic ICM properties. By explicitly following warm ionized, neutral, and molecular gas phases, we studied the complex interplay between AGN activity and the multi-phase ICM. While AGN feedback globally heats the ICM, we find that during the individual AGN jet bursts, hot material is also injected laterally to the jet axis, within the turbulent mixing layer. This material, as it expands, compresses the surrounding hot ICM, reducing the local cooling time, and leads to the formation of cold clumps on a characteristic timescale of $\sim 30$ Myr. By employing tracers, we explicitly track cooling within the affected regions, finding that very hot gas identified in high-compression, low-vorticity zones condenses in situ to form cold clumps. A statistical analysis reveals that the condensation of cold gas is highly promoted once the local turbulent Mach number, $σ_{hot}/c_{s,hot}$, in the hot gas component ($T \geq 10^7$ K) takes values around ~0.3. The presented process is a further important step in understanding the physical mechanisms that lead to the formation of cold gas in the cluster core. Our measured values of the characteristic turbulent Mach number, together with detailed multi-phase gas kinematics predictions, provide important theoretical tools to interpret future X-ray spectroscopy and deep radio data, ultimately to constrain the origin of cool-core cluster nebulae.

Figures (10)

• Figure 1: Top panels: overview of the gas phases of interest during three particular moments of the simulation (columns from left to right). First row: X-ray emission shown with black-red colour tones (see the main text for the emission calculations) and jet tracer projections contours (black colour). X-ray cavities coincide with the locations where jet tracer material is prevalent; second row: Warm ionized gas ($3\times10^3\ \mathrm{K}<T<5\times10^4\ \mathrm{K}$) column density superimposed on the X-ray image, shown with blue-green colour tones; third row: molecular gas column density, superimposed on the previous two images, shown with yellow-orange colour tones. Fourth row: projected velocity along a line-of-sight of the warm ionized gas phase, which is in good agreement with observations. Bottom plot: Total mass of gas with temperature below $5\times10^4\ \mathrm{K}$ (blue curve) and AGN power (grey curve), plotted as a function of time. Purple bands indicate corresponding time instances of the maps shown. The amount of warm and cold gas correlates with the AGN power because the cold gas, accreting onto the central SMBH, ignites the AGN feedback. The AGN-driven jets propagating through the multi-phase ICM create further perturbations, leading to the renewed formation of cold clumps and filaments.
• Figure 2: From left to the right, X-ray emission-weighted profiles of electron number density, temperature and entropy of our simulations, colour coded with respect to time (purple-yellow colour bar), and compare with: the Perseus cluster derived by Churazov04 (green diamonds); cool-core clusters from the ACCEPT sample by Cavagnolo_2008 (grey lines), with a central temperature in a range of $1.5\times10^7\ \mathrm{K} \leq T \leq 5\times10^7\ \mathrm{K}$ and a central entropy less than $50\ \mathrm{keVcm^{-2}}$, i.e. having similar properties to the Perseus cluster. We find that our simulated galaxy cluster profiles are in good agreement with the observed Perseus-like cool-core clusters.
• Figure 3: Total mass of gas with temperature below $5\times10^4\ \mathrm{K}$ (solid blue line), as well as mass of neutral hydrogen $\mathrm{HI}$ (dashed blue line), molecular hydrogen $\mathrm{H2}$ (dotted blue line) and warm-ionized phase (dot-dashed blue line). Note that the total amount refers to both hydrogen and helium, while the different phases refer only to hydrogen. For completeness, the total stellar mass (orange dashed line) is also shown. All the quantities are largely consistent with the values observed in real systems.
• Figure 4: A series of time instances showing the spatial distribution of the jet fraction in the meridional plane (red-yellow colour-coding). The column density of the gas with temperatures below $5\times10^4\ \mathrm{K}$ is overplotted as well (blue-green colour-coding). The black circle indicates a $6\ \mathrm{kpc}$ radius region, centred at $(x, y, z)=(-3, -12, -5)\ \mathrm{kpc}$, in which the properties of Fig. \ref{['fig:hist_hotGas_coolTime']} are computed. As the jet material propagates laterally towards the indicated region it initially drags some of the cold gas with it ($t \sim 1.34-1.38$ Gyr); meanwhile, by injecting turbulence, creates the perturbations (Fig. \ref{['fig:hist_hotGas_coolTime']}) that, in the next $30\ \mathrm{Myr}$, form fresh cold gas in situ (Fig. \ref{['fig:slice_initial_tracer']}).
• Figure 5: Simplified sketch of the cold clumps formation model at the "edges" of the main jet. Only a small vertical section of one jet is shown for simplicity. The red arrow indicates the main velocity of the jet ($\sim 1000\ \mathrm{km/s}$), while the edges are expanding in a perpendicular direction at a lower speed ($\sim 300\ \mathrm{km/s}$). The thin black line demarks the separation of the jet material from the ICM. The green curly regions indicate the presence of turbulence, and the bold black arrows indicate the pressure gradient direction. Turbulent vortices and black arrows marking pressure gradients are shown only in some parts of the sketch for clarity. The blue clouds indicate the new cold clumps that are forming.