Published in the Astronomy and Astrophysics Review, 10, 1, 2000

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THE MOST METAL-POOR GALAXIES

Daniel Kunth and Göran Östlin


Institut d'Astrophysique de Paris (IAP)
98bis Boulevard Arago
75014 Paris, France


ABSTRACT. Metallicity is a key parameter that controls many aspects in the formation and evolution of stars and galaxies. In this review we focus on the metal deficient galaxies, in particular the most metal-poor ones, because they play a crucial rôle in the cosmic scenery. We first set the stage by discussing the difficult problem of defining a global metallicity and how this quantity can be measured for a given galaxy. The mechanisms that control the metallicity in a galaxy are reviewed in detail and involve many aspects of modern astrophysics: galaxy formation and evolution, massive star formation, stellar winds, chemical yields, outflows and inflows etc. Because metallicity roughly scales as the galactic mass, it is among the dwarfs that the most metal-poor galaxies are found. The core of our paper reviews the considerable progress made in our understanding of the properties and the physical processes that are at work in these objects. The question on how they are related and may evolve from one class of objects to another is discussed. While discussing metal-poor galaxies in general, we present a more detailed discussion of a few very metal-poor blue compact dwarf galaxies like IZw18. Although most of what is known relates to our local universe, we show that it pertains to our quest for primeval galaxies and is connected to the question of the origin of structure in the universe. What do QSO absorption lines and known distant galaxies tell us already? We illustrate the importance of star-forming metal-poor galaxies for the determination of the primordial helium abundance, their use as distance indicator and discuss the possibility to detect nearly metal-free galaxies at high redshift from Lyalpha emission.


Table of Contents

INTRODUCTION - WHAT IS A METAL-POOR GALAXY

HOW ARE METALLICITIES MEASURED?
H II Regions
Planetary Nebulae
Photometry of Resolved Stellar Populations
Stellar Spectroscopy
Estimates From Spectroscopy and Photometry of Integrated Light
Othe Methods

WHAT CONTROLS THE METALLICITY OF A GALAXY?
Setllar Evolution and Nucleosynthesis
Star Formation History
Outflows and Inflows
Mergers and Interactions
Mixing
Chemical Evolution Models

THE GENERAL PROPERTIES OF METAL-POOR GALAXIES
Dwarf Irregular Galaxies
Chemical Abundances of Dwarf Irregular Galaxies
Dwarf Elliptical/Spheroidal Galaxies
Chemical Abundances of Local Group dE/dSph Galaxies
Abundances of dEs Outside the Local Group
Low Surface Brightness Galaxies
Chemical Abundances of LSBGs
Blue Compact and H II Galaxies
Morphology and Structure of BCGs
The Age of the Underlying Population
Ongoing Star Formation, Starburts and Star Clusters
The Wolf-Rayet Galaxies
Gas Contents and Dynamics of BCGs
Starburst Triggers in BCGs
The Chemical Abundances of BCGs
Abundance Ratios in BCGs
Summary of BCG Metallicities
Tidal Dwarfs

INDIVIDUAL BONA FIDE METAL-POOR GALAXIES
IZw 18
Chemical Abundance of IZw 18
On the Age of IZw 18
The Companion of IZw 18
SBS0335-052
The Companion of SBS0335-052
Summary

SURVEYS FOR METAL-POOR GALAXIES AND THEIR SPACE AND LUMINOSITY DISTRIBUTIONS
Surveys
Ground-based Selection
Selection Effects at Work
How Metal-Poor Galaxies Can Be Found?
H I Clouds
New Objective-Prism Surveys in Progress
Luminosity Function
The Spatial Distribution of Metal-Poor Galaxies

GLOBAL RELATIONS AND EVOLUTIONARY LINKS
The Metallicity-Luminosity Relation, and Other Empirical Relations
Evolutionary Scenarios and Connections

METAL-POOR GALAXIES, COSMOLOGY AND THE EARLY UNIVERSE
The Primordial Helium Abundance
QSO-Absorption Line Systems
Star Forming Galaxies at High Redshift
Metal Production at High Redshift
The Rôle of Dwarfs in Hierarchical Structure Formation
H II Galaxies as Distance Indicators

SUMMARY AND CONCLUSIONS

ACKNOWLEDGEMENTS

REFERENCES

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