Circulating gas and Filaments in Brightest Cluster Galaxies : a perfect laboratory to understand Galaxy Growth and AGN-feedback
Better characterising the accretion and ejection mechanisms of matter in galaxies in the presence of an active galactic nucleus (AGN) is essential for understanding a significant stage in the life of galaxies: how can star formation be regulated by the activity of the central black hole? The influence of an active galactic nucleus (AGN) on a galaxy’s environment is well observed in the central galaxies of clusters (BCG): enormous bubbles are visible in the intergalactic medium. These bubbles are located in the direction of the radio jet emitted by the AGN: these jets of particles push the hot gas and excavate immense cavities. This interaction produces large amounts of energy capable of globally regulating the cooling of the intra-cluster gas, but also locally triggering gas cooling (energy is efficiently radiated in compressed regions). So, the presence of an AGN induces a circulation of matter. The objective of LYRICS was to characterise the nature and origin of giant molecular filaments observed around central galaxies of clusters. The work of LYRICS confirmed that these filaments are the imprints of the gas circulation induced by the central black hole in BCGs. In the context of this project, we addressed the following questions: (i) what can we learn from multi-wavelength observations of a sample of BCGs? LYRICS showed that filaments are ubiquitous in local relaxed clusters. The size of these filaments is linked to the thermodynamic state of the cooling hot gas (they are only found in dense cores); (ii) what kind of excitation mechanism is capable of producing all the observed emission lines in the filaments? Modelling the excitation of the filaments by the hot gas radiation in clusters (X-rays) reproduces very well a large part of the observed emission lines. LYRICS favours a local excitation mechanism typical of galaxy clusters; (iii) what is the evolutionary trajectory (circulation) of the cooling gas inside the filament? Simulations explain how filaments can form as transient structures with gas being ejected and accreted during active phases of the AGN.
Multi-Wavelength Observations, Photo-Ionisation Modelling and Numerical Simulations
The results of LYRICS were obtained through:
(i) Observations conducted using state-of-the-art instruments on world-class telescopes: We carried out observations of a sample of BCGs in optical wavelengths with integral field spectrographs installed on the Very Large Telescope (VLT, 8-metre diameter) in Chile. All sources had previously been observed in X-rays by the Chandra satellite. A significant portion of this sample was also observed with ALMA (Atacama Large Millimetre Array) with its 50 antennas. The combination of these multi-wavelength observations led to the very first comprehensive study of local cluster central galaxies, as presented in Olivares et al. (2019).
(ii) Modeling of gas excitation by photo-ionisation: We used the CLOUDY code, one of the best-suited codes for this purpose, developed by one of the international collaborators of LYRICS. Model grids involving both X-rays and cosmic rays were produced and analysed. This resulted in a very large number of models that were compared to the emission lines observed in the filaments surrounding BCGs. The key step was the study of the impact of radiation penetration into the depth of the clouds. The addition of this new dimension increased the number of models, making the modelling analysis much more complex but leading to a set of solutions consistent with observations in a parameter space that had not been explored previously, as presented in Polles et al. (2021).
(iii) Numerical simulations: We improved RAMSES, one of the most advanced and comprehensive codes currently used to simulate the formation and evolution of galaxies. A more complete description of AGN jets was implemented, showing how hot gas can condense into cold filaments. These filaments fragment into smaller clouds as observed, see Beckmann et al. (2019). However, overall cooling rates remain relatively high. The role of cosmic rays injected by the AGN was then studied more coherently: they contribute to stabilising the cooling of hot gas but rapidly destroy the central nebula, as described in Beckmann et al. (2022).
Major results
LYRICS has demonstrated that extended and very cold filaments are ubiquitous around the central galaxies of galaxy clusters. The size, mass, and velocity of these filaments are consistent with them being produced by thermal instability within the hot gas surrounding the central galaxy of the cluster. LYRICS has successfully provided a possible explanation for the typical spectral emissions observed in these filaments: X-ray radiation from the hot intra-cluster medium itself, penetrating into dense clouds can reproduce the filament emission observations, with cloud depth being the crucial parameter to explore. Lastly, LYRICS has shown that a self-consistent implementation of jets in numerical simulations can reproduce the observed filaments, despite an excess of cooling that needs further control. An implementation of cosmic rays induced by the jets can solve this issue but has a non-negligible impact on filament formation. The presence of self-directed jets does not help in maintaining radial heat conduction (heat flow from large to small radii), which continues to play an insignificant role in the overall thermal balance of the hot intra-cluster medium.
Figure : The breath of a black hole: Here we are at the centre of a cluster of galaxies in the near Universe. This gigantic cluster, nearly 200,000 light-years across, is bathed in large quantities of burning gas. Over time, some of this gas eventually cools and falls onto a supermassive black hole lurking at the heart of the galaxies. While the collapse of this condensed gas should lead to the formation of new stars, paradoxically this is not the case. The cosmic monster does not engulf everything, but ejects streams of matter and energy at phenomenal speeds. These jets blow immense bubbles around and beyond the galaxy, which, as they dissipate, moderate the cooling of the intergalactic gas. The aim here is to understand the regulating role of black holes in the formation of galaxies as we see them around us today. © Ricarda Beckmann, Institut d’astrophysique de Paris (IAP). https://www.paris-centre.cnrs.fr/fr/evenement/concours-photo-cnrs-la-preuve-par-limage
