emission.
Published in the Astronomy and Astrophysics Review, 10,
1, 2000
For a PDF version of the article, click
here.
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
Ly emission.
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