Galaxies and the magnetization of intergalactic space

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There is increasing evidence that widespread magnetic fields in the intergalactic medium (IGM) originate in galaxies. Up to now the field strengths have been easiest to estimate where the ambient intergalactic (i.g.) nonrelativistic gas density is highest, i.e., in galaxy clusters, the gravitational “sinks” of large scale matter flows. Intracluster gas densities reach 10−3–10−2 cm−3, and magnetic field strength estimates range from 1 to 40 μG. For much of the intracluster volume the plasma β∼1, a consequence of the intracluster magnetic field measurements, and x-ray-determined densities and temperatures of the thermal intracluster (ic) gas. While these ic gas densities represent an impressively good vacuum by terrestrial standards, they are still about 1000× more than in the general IGM. The logical next question is: Are there significant magnetic fields in the wider IGM beyond clusters? Recent, preliminary estimates in i.g. zones beyond, but not far from clusters are 10−6–10−7 G. Integrating this field strength over a typical intergalaxy separation volume gives an average magnetic energy density within the “walls and filaments” of intergalactic space of ∼10−15 B−6.5 ergs cm−3. Interestingly, for Big∼3×10−7 G, and plausible values of nig∼5×10−6 cm−3, and Tig∼108 K—the intergalactic plasma β may be within a factor 10 of unity, roughly comparable to β within the ∼103 times denser medium in galaxy clusters. It is now widely accepted that the only plausible energy source for these intergalactic fields is gravitational, e.g., not the aggregate thermonuclear energy from stars. Two sufficiently large gravitational energy reservoirs are (1) the kinetic energy from large scale intergalactic matter flows, and (2) the gravitational binding energy of supermassive black holes that have been found to be nearly ubiquitous in the nuclei of medium-to-large galaxies. This paper focuses mostly on (2), and also discusses why the largest radio sources are the best independent calibrators, and calorimeters of magnetic energy release into i.g. space from individual galaxy systems.