2.4 Lyman Continuum Optical Depth
An additional, important piece of information about the gas distribution and optical depth comes from studying the wavelength region around the Lyman limit, at 912 Å. This wavelength range has been observed only in high luminosity AGNs and most of these objects do not show any significant Lyman jump in absorption or in emission. A conservative limit on the Lyman discontinuity (the relative change in the continuum level at the Lyman limit), in most of the observed cases, is about 0.1. If the line emitting gas completely surrounds the continuum source this must mean (912 Å) << 1. Alternatively, the gas may be clumped into small clouds, covering only a small fraction of the continuum source. In this case, individual clouds can have large Lyman optical depth without violating the observational limit. The strong observed broad lines of MgII and Fell tend to support the second hypothesis, at least for the broad line gas. Whatever the case may be, it seems that only a small fraction of the total available continuum energy is absorbed by the line emitting gas. More observations are needed to confirm this finding in low luminosity AGNs. This issue is of primary importance for the understanding of AGNs and is further discussed in chapters 6 and 10.
An additional, important piece of information about the relative size of the clouds and the source of continuum emission comes from X-ray studies. It is found that low luminosity AGNs tend to show larger X-ray opacity and more material on the line of sight. Very luminous objects do not show any X-ray absorption. This may or may not be related to the emission line clouds discussed later on.
To summarize, the study of AGN line intensities, variability and profiles reveals information about the physical conditions in the line emitting gas, the gas distribution and dynamics. The optical depth of the gas and the amount of continuum energy absorbed by it, is deduced from observations of the Lyman and the soft X-ray continuum. This information can be used to construct theoretical models for AGNs. The following chapters address all these issues in greater detail.