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The density inside the radio bubbles is much lower than that of the ambient gas, so the bubbles should be buoyant and rise outward in the clusters. These rising bubbles can transport energy and magnetic fields into the clusters.

Evidence of bubbles that have risen buoyantly away from the cluster centers has been found in the Perseus (Fabian et al. 2000) and Abell 2597 (McNamara et al. 2001) clusters. These bubbles are not spatially coincident with the 1.4 GHz (Perseus) or 8.44 GHz (Abell 2597) radio contours from the central AGN, and are farther from the cluster center than the radio emission. These features have been referred to as "ghost bubbles" or "ghost cavities" and are thought to result from a previous outburst of the radio source. Examples of these features are shown in Figures 11 (Perseus, Fabian et al. 2000) and 12 (Abell 2597, McNamara et al. 2001). The buoyancy rise time for the ghost cavities to arrive at their projected positions has been calculated for these objects, and this timescale reveals the repetition rate of the radio outbursts from the AGN, assuming that the cavities result from a previous outburst. For A2597, for example, the repetition rate is approximately 108 yr. This is similar to the cooling time of the central gas, suggesting that a feedback process is operating, where cooling gas fuels the AGN, the AGN has an outburst and heats the gas, then the gas cools and fuels the AGN, etc. (McNamara et al. 2001).

Figure 11

Figure 11. Adaptively-smoothed Chandra image of the center of the Perseus cluster. The inner bubbles that are associated with the current 1.4 GHz radio emission are seen, as well as two ghost cavities to the S and NW of the cluster center. NASA/IoA/A.Fabian et al.

Figure 12

Figure 12. The central region of Abell 2597 as observed by Chandra, after subtracting a smooth background cluster model. The contours of the small central radio source are superposed. Outer ghost cavities are to the NE and SW of the cluster center and are not clearly associated with the 8.44 GHz radio contours shown here. This figure is from McNamara et al. (2001).

Further evidence that the ghost cavities were created by radio lobes earlier in the life of the AGN comes from the detection of low frequency radio emission that is spatially coincident with the outer cavities. The clearest example of this correlation is found in the Perseus cluster when the 74 MHz radio emission is compared with the X-ray emission (Fabian et al. 2002). There also appears to be a similar correlation in the Abell 2597 cluster, based on the comparison of lower frequency radio data than that shown in Fig. 12 and the Chandra image (McNamara et al. 2001).

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