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Perhaps I am late, but the GCNS is available via VizieR. The data can be both queried there and downloaded via FTP: https://cdsarc.cds.unistra.fr/viz-bin/cat/J/A+A/649/A6#/browse, theoretically it is also available via the Gaia-Archive (https://www.cosmos.esa.int/web/gaia/), at least according to an announcement dating 21/12/2021, but I did not manage to ...


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The jump occurs at the redshifted wavelength of the Lyman-$\alpha$ line, so this is the Gunn-Peterson trough, which is caused by neutral hydrogen in the intergalactic medium suppressing any radiation with shorter wavelengths for sufficiently distant quasars. This is a limiting case of the Lyman-$\alpha$ forest formed by the absorption lines caused by all the ...


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