Heating of Galaxy Clusters by Turbulent Energy Dissipation (2005-2008)The observed gas in galaxy clusters is cooling radiatively, with a lifetime less than the Hubble time in many cases. The radiative losses are greatest near the centre, since the gas densities are highest there. Due to the loss of internal energy, in these regions, the gas pressure near the cluster centre also falls. Consequently, the weight of the overlaying gas compresses this gas causing an increase in the density and therefore the radiative losses still further. Without any heating this process would cause large amounts of gas to move towards the gravitational centre of the cluster before condensing into molecular clouds and ultimately forming stars. Since we do not observe such large amounts of excess star formation around the galaxy centre as one may expect, it has been proposed that the gas is heated in some way. There are two main candidates for this heating mechanism. One of these is thermal conduction which transfers energy from large radii to the central regions, where it is required. This mechanism has been found to be sufficient in some 1-d models. However, our 3-d, hydrodynamic models of the Virgo cluster suggest that thermal conduction is likely to be highly suppressed and probably does not contribute strongly to heating the central regions of galaxy clusters. The other proposed mechanism is in the form of outflows from the AGNs, at the centre of the clusters, that inflate bubbles in the intracluster medium. These bubbles must then, presumeably, dissipate their energy somehow. In addition, the thermal conduction and viscosity of the cluster gas may also play a part in dissipating sound waves excited by the buoyant rise of these bubbles. These scenarios cannot be solved analytically and therefore we must use numerical simulations. [1] http://doi.org/10.1111/j.1365-2966.2006.10032.x [2] http://doi.org/10.1111/j.1365-2966.2007.12622.x 
More images of simulation results here. |
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