Galaxy clusters are the most massive collapsed structures in the Universe, with potential wells filled with hot, X-ray-emitting intracluster medium (ICM). Observations, however, show that a substantial number of clusters (the so-called cool-core clusters) also contain large amounts of cold gas in their centres, some of which is in the form of spatially extended filaments spanning scales of tens of kiloparsecs1,2. These findings have raised questions about the origin of the cold gas, as well as its relationship with the central active galactic nucleus (AGN), whose feedback has been established as a ubiquitous feature in such galaxy clusters3,4,5. Here, we report a radiation-hydrodynamic simulation of AGN feedback in a galaxy cluster, in which cold filaments form from the warm, AGN-driven outflows with temperatures between 104 and 107 K as they rise in the cluster core. Our analysis reveals a new mechanism that, through the combination of radiative cooling and ram pressure, naturally promotes outflows whose cooling times are shorter than their rising times, giving birth to spatially extended cold gas filaments. Our results strongly suggest that the formation of cold gas and AGN feedback in galaxy clusters are inextricably linked and shed light on how AGN feedback couples to the ICM.
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The data that support the plots within this paper and other findings of this study are available from the corresponding author upon reasonable request.
The simulations presented here were performed using an adapted version of the Enzo code. The main repository is available at http://enzo-project.org/, and the customizations made for this work are available from the corresponding author upon request.
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Y.Q. thanks L. C. Ho and S. M. Faber for useful discussions. Y.Q. acknowledges support from the National Key R&D Program of China (2016YFA0400702), the National Science Foundation of China (11721303, 11991052), and the High-Performance Computing Platform of Peking University. T.B. thanks the Kavli Institute for Theoretical Physics, where one portion of this work was completed, for its hospitality. Support for this work was in part provided by the National Aeronautics and Space Administration through Chandra Award Number TM7-18008X issued by the Chandra X-ray Center, which is operated by the Smithsonian Astrophysical Observatory for and on behalf of the National Aeronautics Space Administration under contract NAS803060. This research was supported in part by the National Science Foundation under grant number NSF PHY-1748958, and in part through research cyberinfrastructure resources and services provided by the Partnership for an Advanced Computing Environment (PACE) at the Georgia Institute of Technology, Atlanta, Georgia, USA.
The authors declare no competing interests.
Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.
The total AGN luminosity is allocated to kinetic (blue) and radiative (orange) luminosity as a function of SMBH accretion rate. Lighter coloured bands correspond to the instantaneous AGN luminosities and darker lines show luminosities smoothed over 0.2 Myr. Dotted vertical lines mark the instances in time corresponding to the panels in Fig. 1a–c. Inset shows the evolution of the AGN luminosity over 10 Gyr in the parent simulation, smoothed over ~0.1 Gyr. The grey vertical strip marks the duration of the high-cadence re-simulation.
Extended Data Fig. 2 Instantaneous radial velocity distribution for outflows in the high-cadence simulation.
Outflow speed as a function of radius, colour-coded in terms of the mass-weighted temperature (a) and neutral hydrogen mass (b). The bulk of the cold gas is traveling with speeds below 1,000 km s−1, in agreement with observations and with predictions of the analytic model. The time of this snapshot corresponds to Fig. 1b.
Extended Data Fig. 3 Velocity dispersion of different temperature phases of the gas in simulated cluster core.
The resulting velocity dispersion is calculated as the average of the x, y, and z components, each weighted by the X-ray emissivity (for hot and warm gas) and Hα emissivity (for cold gas), at radii between 3 and 20 kpc (see Supplementary Information). While warm and cold gas share similar values, the velocity dispersion of the hot gas shows stronger, uncorrelated variability, indicating a separate kinematic origin from the cold gas filaments.
a–c, Projected gas density showing the spatial reach of the AGN-driven material, traced by a passive fluid injected from the central 1 kpc, corresponding in time to Fig. 1a–c. A movie showing the evolution in the high-cadence simulation is provided in Supplementary Video 1.
a, The size of the smallest dust grain that can survive in the warm outflows as a function of their initial temperature. For a characteristic dust grain size of 1 μm or less, most dust content is eliminated in outflows with initial temperature higher than 4 × 106 K. b, Radial temperature distribution of the tracer fluid, colour-coded with the thermal dust sputtering rate, weighted by cell mass. For dust size a ~ 0.1 μm, blue colour corresponds to a sputtering timescale of tsp ≳ 1 Myr. This is longer than the cooling time of the warm outflows, indicating that dust can be preserved in the outflowing gas.
Supplementary discussion and references.
Formation of cold gas filaments from warm galactic outflows driven by AGN feedback. Left: 20-Myr evolution of the warm and cold outflows, projected along the line of sight. Orange (grey) colour intensity represents the column density of ionized (neutral) hydrogen for warm (cold) gas, same as in Fig. 1a–c. Right: spatial expansion of the tracer fluid injected from the central 1 kpc at the start of the simulation. Colour intensity shows the projected gas mass density of computation cells that contain the tracer fluid. Visual comparison with the left movie indicates that cold filaments contain gas launched from the centre of the cluster.
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Qiu, Y., Bogdanović, T., Li, Y. et al. The formation of dusty cold gas filaments from galaxy cluster simulations. Nat Astron (2020). https://doi.org/10.1038/s41550-020-1090-7