The first X-rays observations in the 70’s revealed that galaxy clusters are dominated by diffused very hot gas (more that 10 millions degrees). At these temperatures, the hot gas loses a huge amount of energy by thermal emission and cools. In the most massive clusters, the electronic density is so high, that the cooling time of the hot gas is much smaller that the age of the universe. Under these conditions, the gas is not in hydrostatic equilibrium and flows slowly towards the galactic center. These clusters are named "cooling flow clusters". If theory predicts large amounts of cooling gas, observations fail to find as much gas as predicted in the temperature range between one and 10 millions degrees, leading to a difficult problem. A solution to this problem could be the quenching or damping of the cooling due to the heating produced by the central active galactic nuclei (AGN). An AGN is effectively present at the center of all cooling flow clusters.
Recent observations of H-alpha emission of relatively cold gas (10 000 degrees) and very cold gas (tens of degrees) traced by the emission of the CO molecule, showed that gas with low temperature is present in the atmosphere of those clusters. However, the gas mass derived from these observations is 10 times below the predictions of the simpler model (without AGN). The spatial distribution of this gas is surprising (see Fig. 1). The gas forms filamentary structures, spread all around the cluster center.
But the link between the cluster cooling and the presence of the cold gas if far from obvious. Those filaments are very extended (more that 200 000 light years for the longest) and cross regions with cooling times differing by more than an order of magnitude. Moreover, those filaments have a peculiar velocity, indicating that they are stretching (the gas seems to raise at the top of the filaments, while it is falling and being accelerated more and more near the cluster center).
Using numerical simulations (N-body/hydrodynamics) at very high resolution, it has been possible to propose a coherent scenario at the origin of the cold filaments. The central AGN generates a supersonic jet, blowing up plasma bubbles of very high temperature (hotter than 100 millions degrees). Those hotter but less dense bubbles (compared to the ambient plasma), migrate upwards, due to the Archimede’s force. During the migration, a fraction of the ambient gas is dragged by the bubbles at higher radius.
During the migration that lasts more than 600 million years, this gas which cools relatively rapidly (in 400 million years) , has time to cool below one million degrees. At this temperature it is no longer supported by the pressure and consequently falls towards the cluster center, forming a filamentary structure below the bubble (see Fig. 2). The increase of its density reinforces its cooling and its temperature quickly falls below 10 000 degrees. The observed stretching of the filaments is well reproduced by the simulations. The top of the filament is still entrained by the bubble and moves away from the center, while the bottom is nearly in free fall towards the center.
In summary, if the central AGN provides heating and contributes to the global quenching of the cluster cooling, it is also responsible for the production of cold gas in outlying regions.
The team is composed of : Yves Revaz (1), Françoise Combes (1), Philippe Salomé (2) (1) LERMA, Observatoire de Paris ; (2) IRAM, Grenoble