Heating by a Central AGN

The prerequisite for avoiding massive cooling and condensation is a suitable heat source that is fine-tuned to just compensate the radiative losses in the cool core. This is, for example, achieved by the cycle of feeding a central AGN with cooling flow gas until the energy output of the AGN is limiting (regulating) the further cooling of the ICM, such that the gas condensation rates are reduced by factors of probably tens to hundreds [15,34,35]. In fact, high resolution Chandra images now show various examples of interaction effects of central AGN with the cluster ICM providing direct evidence of energy input into the radiatively cooling ICM (e.g. [7,55,64,99]). The most prominent example of AGN-ICM interaction is that of the AGN in NGC1275 in the Perseus cluster of which Chandra images are shown in Fig. 23.16, [55]. Here and in now about 20 other cases we observe how the relativistic plasma ejected by the AGN in form of jets fills radio lobes, which push the X-ray emitting ICM away and forms X-ray underluminous cavities.

The observed interaction effects and the derived properties of the ambient ICM allow us to roughly estimate the power of the mechanical energy injection of the AGN and one finds values in the range of 1044-1045 erg s~\ in some cases much more than the energy that is radiated in the cooling core region. The most important question that has still to be solved is how the energy from the radio lobes is transferred to the ICM in detail. The heating has to be fine-tuned globally such that cooling is prevented effectively throughout the cooling area. How this is managed is far from clear, but recent observations of possible sound waves ("ripples" in the

Fig. 23.16 Left: Chandra X-ray image of the central region of the Perseus cluster around the dominant elliptical galaxy NGC 1275 with radio contours superposed (Fabian et al. [55]). The region covered by the radio lobes show clear cavities in the X-ray surface brightness distribution where the X-ray emitting ICM has been displaced by relativistic synchrotron emitting plasma. Right: X-ray image of the same region produced by unsharp masking emphasizing regions of high local contrast. The concentric structures ("ripples") revealed by this picture are interpreted as sound waves or weak shock waves produced by the interaction of the expanding radio bubbles and the ambient ICM [55]

Fig. 23.16 Left: Chandra X-ray image of the central region of the Perseus cluster around the dominant elliptical galaxy NGC 1275 with radio contours superposed (Fabian et al. [55]). The region covered by the radio lobes show clear cavities in the X-ray surface brightness distribution where the X-ray emitting ICM has been displaced by relativistic synchrotron emitting plasma. Right: X-ray image of the same region produced by unsharp masking emphasizing regions of high local contrast. The concentric structures ("ripples") revealed by this picture are interpreted as sound waves or weak shock waves produced by the interaction of the expanding radio bubbles and the ambient ICM [55]

ICM [55]) observed in the Perseus cluster and shock waves observed for example in M87 (Fig. 23.17) and Hydra A [64,90] suggest that these waves, created by the expanding radio bubbles, are the main agent to transfer the energy from the relativistic to the thermal plasma. A large effort is now also going into detailed simulations of these processes (e.g. [26]).

0 0

Post a comment