So far we have only considered line intensities and line profiles in individual objects. More insight on AGNs can be gained by comparing different objects, and by analyzing the statistical properties of large samples. It enables us to search for luminosity dependences, to compare bright and faint objects in a variety of ways, and to examine the consequences of the photoionization theory.
In this chapter we discuss several observed correlations between the broad emission lines and the nonstellar continuum of AGNs. We use them to obtain further estimates on the ionization parameter, the covering factor, the velocities and the masses of AGNs. Thin accretion disks are also discussed. A parallel discussion, based on observations of the narrow lines, is given in chapter 11. Throughout the chapter we use the symbol L to designate the integrated continuum luminosity over some specified frequency range, and L the monochromatic luminosity per Å.
10.1. Line Intensity vs. Continuum Luminosity
The intensity of most broad emission lines is strongly correlated with the continuum luminosity. An example is shown in Fig. 28, where the L intensity of 328 AGNs is compared with their continuum luminosity. An almost perfect correlation, with a best slope of 0.88, is found. This gives further support to the idea that photoionization is, indeed, the main source of excitation for the lines.
Figure 28. L luminosity vs. continuum luminosity for 328 AGNs. (Here and in the following examples H0 = 75 km s-1 Mpc-1 and q0 = 0.5).
The slope of the correlation in Fig. 28 is close to, but not exactly 1.0. This can be interpreted in several ways. One possibility is that the integrated covering factor decreases with increasing luminosity. If this is correct, and there are no other factors involved, then the decrease in covering factor amounts to a factor of about 3.5 over the range of 104.5 in continuum luminosity. Another possibility is a change in the shape of the ionizing continuum, such that the number of ionizing photons (mainly in the 1-4 Ryd. range), compared with L1215, is smaller in brighter objects; i.e. a softer ionizing continuum in more luminous objects. The change must be small, as the relative intensity of the high excitation lines does not seem to be a function of luminosity. There are other possibilities, such as changes in the amount of the optically thin material and/or the optical depth of the clouds, with luminosity.