Stars can be sorted into about half a dozen categories using their individual values for two basic parameters to determine their location on the Hertzsprung-Russell diagram. The importance of this classification derives in part from its observational simplicity, but especially from the underlying theoretical framework which allows one to deduce many of the likely physical characteristics, including the evolutionary status of a star directly from the observable characteristics. It is with such an elegant and powerful scheme in mind that we should attempt to classify the absorption line systems of QSOs.
The absorption systems would obviously be expected to include a wide variety of species, simply because absorption will arise in any and all gaseous structures which are numerous, spatially extended, or located preferentially close to QSOs. Great variety is indeed being found, but unfortunately current observations are only sufficient to provide an extremely restricted range of parameters which can be used in a classification scheme. The scheme which has been devised has become the starting point for all discussions of the systems, and consequently it is vital that its strengths and weaknesses be thoroughly understood.
The velocity distribution of the gas causing the absorption is regarded as the primary characteristic. Systems which have a velocity spread in excess of about 2000 km s-1 are known as the trough or broad absorption lines (BAL) systems. They display a high level of ionization, are frequently very complex, and are most naturally interpreted as arising in gas ejected from QSOs. Nearly all of the remaining systems have velocity widths of under 200 km s-1. Statistical analyses of the distribution of these narrow line systems indicates that they must predominantly arise in intervening material which is at cosmological distances from the QSOs. This distinction into intervening and ejected systems is seriously incomplete because systems with intermediate properties are found, and we do not know the minimum velocity width for ejected systems.
A second major characteristic, used in the classification of the narrow line systems, is the presence or absence of absorption due to heavy elements. The 1% of systems which do show heavy elements are known as the metal line systems, while the remaining 99% are the Ly- systems. These two types of systems do differ in several observational respects, but the reasons for these differences are not clearly understood. Whereas it is widely considered that they are two totally separate populations, I wish to stress that much can be gained by examining the differences and similarities. We will see that the questions that arise when one examines the proposition that they are, in fact, closely related are central to any thorough understanding of their nature. The comparative study of the system properties which follows shows that the observational parameters which are used to classify systems are far from ideal. It also highlights the importance of seeking a general framework in which the properties of all narrow lined systems can be understood.