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Deep in the cores of “relaxed” clusters, another problem is brewing. In this high density region the X-ray radiative cooling times of the hot ICM are short enough that 100s to 1000s of solar masses of material should be forming new stars each year. Observations indicate that only 10s of solar masses are turning on each year, and newer observations reveal cool molecular material — all exciting, but far from enough to meet the expectations.

Another view of the same issue is provided by the trends of what cluster astronomers call “entropy,” K = T / n2/3, a quantity that is relatively straightforward both to measure as a function distance from the cluster center, and to interpret. While T and n are both regulated by global cluster parameters such as mass and size, they aren't straightforward indicators of the energy flows. But when combined in this entropy form they are sensitive to the heating and cooling history of the ICM. In the case of successive accretion onto a cluster of increasing mass, the expected entropy profile is K(r) = K0 r1.1 , close to what is observed beyond the cluster cores.

Figure 7

Figure 7. Entropy profiles become shallower in inner cluster regions, mostly due to AGN heating [9].

In the cores, the entropy drops more slowly, requiring some input of heat to the ICM, as shown in Figure 7. Ejections of relativistic plasma from the AGN of the bright central galaxy are one key source of this energy. Figure 8 shows the synchrotron lobes from a radio galaxy having excavated enormous cavities in its surrounding ICM. Episodic injections of up to 1055 J are observed, sometimes near the cluster core, and sometimes at much larger distances. The work done by such outbursts, as well as CR heating and sloshing motions appears sufficient to balance the cooling. This opens up the exciting possibility that clusters are self-regulating over cosmic times. In this scenario, the infall of material onto the central supermassive black hole is controlled by the cooling and instabilities near the cluster core which are themselves controlled by the energy released from the AGN outflows.

Figure 8

Figure 8. The inner 700 kpc of the MS0735.6+7421 cluster combining the X-ray (blue), i-band (white), and radio wavelengths (red) [10].

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