1.1. Some Basic Concepts
For many years our knowledge of the distant universe relied almost exclusively on QSO absorption lines. It is only relatively recently that we have learnt to identify directly `normal' galaxies; up until 1995 the only objects known at high z were QSOs and powerful radio galaxies. The technique of QSO absorption line spectroscopy, illustrated in Figure 1, is potentially very powerful. As we shall see, it allows accurate measurements of many physical properties of the interstellar medium (ISM) in galaxies and the intergalactic medium (IGM) between galaxies. The challenge, however, is to relate this wealth of data, which refer to gas along a very narrow sightline, to the global properties of the absorbers. In a sense, all the information we obtain from QSO absorption line spectroscopy is of an indirect nature; if we could detect the galaxies themselves, our inferences would be on a stronger empirical basis.
Figure 1. The technique of QSO absorption line spectroscopy is illustrated in this montage (courtesy of John Webb). QSOs are among the brightest and most distant objects known. On the long journey from its source to our telescopes on Earth, the light from a background QSO intercepts galaxies and intergalactic matter which happen to lie along the line of sight (and are therefore at lower absorption redshifts, zabs, than the QSO emission redshift, zem. Gas in these structures leaves a clear imprint in the spectrum of the QSO in the form of narrow absorption lines. The task of astronomers working in this field has been to relate the characteristics of the absorption lines to the properties of the intervening galaxies which are normally too faint to be detected directly.
In deriving chemical abundances in QSO absorbers and high redshift galaxies, we shall make use of some of the same techniques which are applied locally to interpret the spectra of stars, cool interstellar gas and H II regions. These methods are discussed extensively in other articles in this volume, particularly those by Don Garnett, David Lambert, and Grazyna Stasinska, and will therefore not be repeated here. The derivation of ion column densities from the profiles and equivalent widths of interstellar absorption lines is discussed in a number of standard textbooks, as well as a recent volume in this series (Bechtold 2002).
When measuring element abundances in different astrophysical environments, we shall often compare them to the composition of the solar system determined either from photospheric lines in the solar spectrum or, preferably, from meteorites. The standard solar abundance scale continues to be refined; here we adopt the compilation by Grevesse & Sauval (1998) with the recent updates by Holweger (2001). We use the standard notation [X/Y] = log(X/Y)obs - log(X/Y) where (X/Y)obs denotes the abundance of element X relative to element Y in the system under observation - be it stars, interstellar gas or the intergalactic medium - and (X/Y) is their relative abundance in the solar system.
Furthermore, it is important to remember that when element abundances are measured in the interstellar gas of the Milky Way, it is usually found that [X/H] < 0. This deficiency is not believed to be intrinsic, but rather reflects the proportion of heavy elements that has condensed out of the gas phase to form dust grains (and therefore no longer absorbs starlight via discrete atomic transitions). As discussed in the review by Savage & Sembach (1996), the `missing' fraction varies from element to element, reflecting the ease with which different constituents of interstellar dust are either incorporated into the grains or released from them. In particular, O, N, S, and Zn show little affinity for dust and are often present in the gas in near-solar proportions; Si, Fe and most Fe-peak elements, on the other hand, can be depleted by large and varying amounts depending on the physical conditions - past and present - of the interstellar clouds under study.
Unless otherwise stated, we shall use today's `consensus' cosmology (e.g. Turner 2002) with H0 = 65km s-1 Mpc-1 (and hence h = 0.65), baryons = 0.022 h-2, M = 0.3, and = 0.7. Table 1 shows the run of look-back time with redshift for this cosmology. Note that with the above cosmological parameters the age of the universe is 14.5Gyr, consistent with recent estimates of the ages of globular clusters (Krauss & Chaboyer 2001). When in these lecture notes we refer to `high' redshifts we usually mean z = 3 - 4 which correspond to look-back times of 12-13 Gyr, when the universe had only 15-10% of its present age. Epochs when z was 1 are generally referred to as `intermediate' or `low' redshifts, even though they correspond to look-back times of up to about 60% of the current age of the universe.
|Redshift||Lookback Time (Gyr)||Lookback time (t/t)|