High Redshift Galaxies

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 Stasiriska, 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). 

The Lyman break technique has been very successful at finding high redshift galaxies... 

Observations of distant objects, notably high-redshift star-forming ( Lyman-break ) galaxies and absorption line systems on the line of sight to quasars, give some information on chemical evolution at epochs not too far from when the first stars and most galaxies were presumably formed. Other information comes from two related effects ... 

Abundance measurements at high redshifts complement compositional studies of stars in our own Galaxy. These new measurements are like samples taken from different cosmic epochs. As we might have expected, they show an overall trend towards metal enrichment. However, the data are as yet fragmentary and fitful (see Chapter 8). 

For comparison, the primordial baryon fraction from observations at high redshift is 15%. Presumably these baryons were present initially, when the galaxy formed. Indeed modelling of disk formation requires an initial baryon fraction of 10 — 15% in order for sufficient cooling to have occurred to form the disk. The missing" galactic baryons amount to a baryon fraction comparable to what is observed, namely around 5 — 10% of the dark halo. 

Figure 18. N/O abundance ratios in spiral and irregular galaxies. Open circles Garnett 1990 open diamonds Thuan et al. 1995 filled circles Garnett et al. 1999 filled squares Garnett Kennicutt 1994, Torres-Peimbert et al. 1989 plus signs Diaz et al. 1991 stars high-redshift absorption line systems from Lu, Sargent Barlow 1998. Note the very low N/O in some high-redshift systems.

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