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

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


THE LYMAN ALPHA FOREST IN THE SPECTRA OF QSOS

Michael Rauch


Astronomy Department, California Institute of Technology
Pasadena, CA 91125, USA
mr@astro.caltech.edu


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


Abstract. Observations of redshifted Lyman alpha 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 Lyalpha absorption cross-section. Comparisons between large data samples from the new generation of telescopes and artificial Lyalpha forest spectra from cosmological simulations promise to become a useful cosmological tool.


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
Lyalpha 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 Lyalpha Forest
The Lyalpha Forest as a Cosmological Laboratory

THE LYMAN ALPHA FOREST AND GALAXIES
The Low Redshift Lyman Alpha Forest
Metal Enrichment In The High Redshift Lyalpha Forest

THE HELIUM LYMAN ALPHA FOREST

PROSPECTS

REFERENCES

For a postscript version of the article, click here.

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