10.2. Line - Line Correlations
Emission line ratios are the best ionization parameter indicators, and the correlation of line ratios with the continuum luminosity can be used to check the U vs. L dependence.
Fig. 29 shows the CIV1549 / L line ratio, as a function of the continuum luminosity, for a sample of 165 AGNs. The scatter in this ratio is large, but the tendency is for the line ratio to decrease with increasing continuum luminosity. This indicates, perhaps, that the ionization parameter in bright quasars is smaller than in Seyfert 1s (Fig. 8). Although the correlation is statistically significant, it is not clear how representative it is of the AGN population. The objects under study were randomly selected from the literature, and there are several potential selection effects to be considered. Moreover, despite the tendency of decreasing CIV1549 / L with increasing luminosity, some bright quasars are definitely exceptional in this respect.
Figure 29. CIV1549 / L vs. continuum luminosity at 1549Å for 165 AGNs.
Line ratio diagrams have been constructed for other emission lines, some in carefully selected samples, where selection effects are not likely to dominate. Such well selected samples are rare, and the number of objects in them rather small. They cover a limited range in luminosity, and general tendencies are hard to discover. Of the more interesting findings we note the increase of the CIII]1909 / CIV1549 line ratio with L, suggesting, perhaps, a change in U and/or density with luminosity.