Abstract
Galaxies do not exist in isolation, either from one another or from their gaseous environments. Unless galaxy formation was implausibly efficient, we might expect some ordinary matter to be left over, still filling the vast intergalactic spaces. Something closely analogous happens when stars are formed. Outflows, whether collimated into narrow jets or in expansive winds, are important to the process, and it appears that the mass of material ejected from a star-forming cloud is roughly equal to that in the final star. A similar rough balance may hold for galaxies and gas which has been expelled from them, though the physics must be quite different. The hot gas bound in clusters of galaxies has shown that not only was there leftover matter, but that this matter was enriched by material expelled from galaxies after being processed through massive stars. Rather than the empty void we might once have thought of, intergalactic space, like interstellar space, turns out to be a lively and important region. C.S. Lewis might almost have been expressing this shift in view-point when he wrote in Out of the Silent Planet that “... the very name Space seemed a blasphemous libel for this empyrean ocean of radiance ...”.
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(2007). The intergalactic medium. In: The Road to Galaxy Formation. Springer Praxis Books. Springer, Berlin, Heidelberg. https://doi.org/10.1007/978-3-540-72535-0_6
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DOI: https://doi.org/10.1007/978-3-540-72535-0_6
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