Intergalactic shock waves

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Young galaxies as well as quasars and active galactic nuclei, have long been considered as possible sources of shock waves entering the intergalactic medium1–4. Following convincing evidence5 that the narrow absorption lines seen shortward of the Lyα emission line in very distant quasars are mostly intervening (and not intrinsic) Lyα absorption lines, various attempts6–8 have been made to relate the cold H I absorbers (in part, at least) with actual shock waves in the intergalactic medium. Here, we consider intergalactic shocks in a low density universe when energy losses from shocks are negligible at the redshifts of interest. However, even at this essentially adiabatic evolution stage, the absorption of the outer radiation in the outermost densest and coldest regions of the shock waves is capable of producing measurable Lyα absorption lines. Moreover, these lines should be observed, due to a specific geometry, in the form of special ‘doublets’. We demonstrate that there are such doublets in quasar spectra among the Lyα ‘forest’, and this can apparently be regarded as the first conclusive evidence in favour of intergalactic shock waves.