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The Lyman α forest

The Lyman α forest - QSO A sample of...

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The Lyman α forest: All QSOs with redshifts large enough for us to observe shortward of Lyman α (1216 Å) in the QSO frame show numerous sharp absorption features apparently due to the presence of intergalactic clouds absorbing in the strongest hydrogen resonance line. This was reported first by Lynds 1971 (ApJLett 164, L73). Reviews on the Lyman α forest include Sargent and Boksenberg 1983, in Quasars and Gravitational Lenses , proc. 24th Liege Astrophysical Colloquium, p. 518; Weymann, Carswell, and Smith 1981 ARA&A 19, 41; and the chapters by Hunstead, Carswell, and Ostriker in QSO Absorption Lines: Probing the Universe (STScI: Cambridge, 1988). Do not confuse Lyman α absorption systems with the metal-line systems identified with "normal" intervening galaxies, or with the broad-absorption line systems that are intrinsic to the
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Unformatted text preview: QSO. A sample of the Lyman α forest is given in Fig. 1 of Young et al. 1982 (ApJ 252, 10, courtesy of the AAS): The strong broad emission feature is Lyman α at the QSO redshift z =1.899. Identification of the narrow features as the same transition is supported by the gross asymmetry on either side of the emission peak. Note that the density of absorption features changes with redshift (wavelength). Adopting some cosmological model, this gives the time evolution of the absorbers. It is traditional (though by now somewhat misleading) to identify distinct absorption features as physical clouds. These clouds either are or are imbedded in the IGM. What can we tell about their properties?...
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