Annu. Rev. Astron. Astrophys. 1998. 36: 267-316
Copyright © 1998 by . All rights reserved

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Although most of the observational basis and many theoretical aspects of Lyalpha forest absorbers had already been established over the past two decades, high signal-to-noise spectroscopy of QSOs with large telescopes, the extension of the wavelength range into the far UV with satellites, and the success of hydrodynamic cosmological simulations have begun to turn the study of the Lyalpha forest from a somewhat esoteric appendix of cosmology into a cosmologically useful tool. If our current understanding is correct, the high redshift Lyalpha forest absorption is the observational signature of most of the baryons throughout most of the history of the universe. Perhaps most importantly, we are looking at the typical fate of matter, without any reference to luminous objects.

The new general picture of the forest that has emerged is still in need of more secure observational conformation, before we can trust any quantitative cosmological conclusions. The simulations agree with some aspects of the data, but are they unique, and consistent with each other ? More detailed comparisons with the data are called for, partly with new techniques of data analysis tailored to maximize the discriminative power. Semi-analytical work still has an important function in guiding our understanding of the actual physics, and in exploring parameter space quickly. The observers need to explore the limitations of the observational techniques, and systematic effects in the data, formerly gracefully hidden by a veil of noise, but now exposed by large mirrors to the strict eye of numerical theory.

We can expect new observational facilities to enlarge the scope of absorption line studies considerably. The projected Cosmic Origins Spectrograph (COS) will increase the spectroscopic efficiency of the HST, benefitting almost all of the observational areas mentioned here. The Sloan Digital Sky Survey (SDSS) should produce large numbers of QSOs useful for spectroscopic follow-up, to study (for example) the large scale structure from intergalactic absorption in three dimensions. Even low resolution QSO spectra will be useful for cosmological purposes. And the use of large optical telescopes is inevitable for studies of gravitationally lensed QSOs, the absorber-galaxy connection at any redshift, and of course for most projects involving narrow, metal absorption lines, a subject of increasing relevance for our understanding of the process of galaxy formation.

Unfortunately, other applications of absorption lines like the remarkably militant debate about the deuterium to hydrogen ratio, the whole topic of intervening metal absorption systems and its relevance for galaxy formation, and the process of reionization could not be treated here. Perhaps we have ignored them with some justification; these subjects are currently in such a state of flux that any review could be out of date before going to press.


I am grateful to Bob Carswell, Martin Haehnelt, Jordi Miralda-Escudé, and Wal Sargent for reading earlier drafts, to Naomi Lubick and Allan Sandage for their thorough editorship, and to NASA for support through grant HF-01075.01-94A from the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS5-26555.

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