Many of these are also Lyman

Many of these are also Lyman - AAS Lanzetta found that...

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Many of these are also Lyman-limit systems , in which the optical depth at the Lyman edge is unity or greater. These are recognized by a step down in flux at the Lyman edge at the absorber (not QSO) redshift, frequently to zero detectable flux. The Milky Way is a Lyman limit absorber from our vantage point in every dirction at z =0. Various treatments are given by Tytler (1982 Nature 298, 427), Bechtold et al 1984 (ApJ 281, 76) and Lanzetta (1988 ApJ 332, 96). An example is shown by Sargent, Steidel, and Boksenberg (1989, ApJSuppl 69, 703, courtesy of the
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Unformatted text preview: AAS): Lanzetta found that these systems seem to have constant comoving space density, and that the Mg II metal-line systems seem to be disappearing relative to Lyman-limit systems. These systems may also be sought in BL Lac objects, avoiding confusion with emission-line or BAL structure but introducing the problem that we may not know the AGN redshift except as the redshift at which the Lyman α forest disappears....
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