Magnetic Energy of the Intergalactic Medium from Galactic Black Holes

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Abstract

A quantitative analysis of two radio source samples having opposite extremes of ambient gas density leads to important new conclusions about the magnetic energy in the IGM. We conclude that giant sources in rarefied IGM environments, which contain magnetic energies E_B ~ 10^60-61 ergs, can be viewed as important "calorimeters" of the minimum energy a black hole (BH) accretion disk system injects into the IGM. In contrast to the radiation energy released by BH accretion, most of the magnetic energy is "trapped" initially in a volume, up to ~10^73 cm^3, around the host galaxy. But since these large, Mpc scale radio lobes are still overpressured after the AGN phase, their subsequent expansion and diffusion will magnetize a large fraction of the entire IGM. This suggests that the energy stored in intergalactic magnetic fields will have a major, as yet underestimated effect on the evolution of subsequently forming galaxies. Comparison with the second sample, consisting of sources within 150 kpc of rich cluster cores, shows that the minimum magnetic energy E_B can be a strongly variable fraction of the inferred accretion energy E_acc, and that it depends on the ambient IGM environment. AGNs inject significant energy as PdV work on the thermal ICM gas, and their magnetic energy, even ignoring the contribution from stellar and starburst outflows, is sufficient to account for that recently found beyond the inner cores of galaxy clusters. Other loss processes in the course of the lobe expansion are considered. We conclude that the aggregate IGM magnetic energy derived purely from galactic black holes since the first epoch of significant galaxy BH formation is sufficiently large that it will have an important influence on the process of both galaxy and visible structure formation on scales up to ~ 1Mpc.

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