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

Reprinted with kind permission from Annual Reviews, 4139 El Camino Way, Palo Alto, California, USA.

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Abstract. Observations of redshifted Lyman forest absorption in the spectra of quasistellar objects (QSOs) provide a highly sensitive probe of the distribution of gaseous matter in the universe. Over the past two decades optical spectroscopy with large ground-based telescopes, and more recently ultraviolet spectroscopy from space have yielded a wealth of information on what appears to be a gaseous, photoionized intergalactic medium, partly enriched by the products of stellar nucleosynthesis, residing in coherent structures over many hundreds of kiloparsecs.

Recent progress with cosmological hydro-simulations based on hierarchical structure formation models has led to important insights into the physical structures giving rise to the forest. If these ideas are correct, a truely inter- and proto-galactic medium [at high redshift (z ~ 3), the main repository of baryons] collapses under the influence of dark matter gravity into flattened or filamentary structures, which are seen in absorption against background QSOs. With decreasing redshift, galaxies forming in the denser regions, may contribute an increasing part of the Ly absorption cross-section. Comparisons between large data samples from the new generation of telescopes and artificial Ly forest spectra from cosmological simulations promise to become a useful cosmological tool.

Keywords: QSO absorption lines, intergalactic medium, galaxy formation, cosmology, UV background radiation

Table of Contents

• INTRODUCTION
• OBSERVATIONAL APPROACHES
• Technical Possibilities, Observational Constraints
• Low Resolution Spectroscopy: Mean Absorption
• Intermediate Resolution Spectroscopy: Line Counting
• High Resolution Spectroscopy: Voigt Profile Decomposition
• OBSERVATIONAL RESULTS
• Redshift Evolution of the Lyman Alpha Forest
• The Proximity Effect: Measuring the Ionizing UV Background
• Absorption Line Widths
• The Column Density Distribution
• Spatial Structure along the Line of Sight: Clustering and Voids
• Spatial Structure Across the Sky: Multiple Lines of Sight
• THE FIRST GENERATION OF MODELS
• Ly Clouds confined by the Pressure of an Intercloud Medium
• Gravitational Confinement: Selfgravity
• Gravitational Confinement: Clouds Dominated by Hot Dark Matter Gravity
• Gravitational Confinement: Cold Dark Matter Minihalos
• THE COSMOLOGICAL SIGNIFICANCE OF THE LYMAN ALPHA FOREST
• Hydrodynamic Simulations of the Ly Forest
• The Ly Forest as a Cosmological Laboratory
• THE LYMAN ALPHA FOREST AND GALAXIES
• The Low Redshift Lyman Alpha Forest
• Metal Enrichment In The High Redshift Ly Forest
• THE HELIUM LYMAN ALPHA FOREST
• PROSPECTS
• REFERENCES