Continuum and emission-line strength relations for a large active galactic nuclei sample

We report on the analysis of a large sample of 744 type 1 active galactic nuclei, including quasars and Seyfert 1 galaxies across the redshift from 0 less than or similar to z less than or similar to 5 and spanning nearly 6 orders of magnitude in continuum luminosity. We discuss correlations of continuum and emission-line properties in the rest-frame ultraviolet and optical spectral ranges. The well-established Baldwin effect is detected for almost all emission lines from O VI lambda1034 to [O III] lambda5007. Their equivalent widths are significantly anticorrelated with the continuum strength, while they are nearly independent of redshift. This is the well-known Baldwin effect. Its slope beta, measured as log W-lambda proportional to beta log lambdaL(lambda) (1450 Angstrom), shows a tendency to become steeper toward higher luminosity. The slope of the Baldwin effect also increases with the ionization energy needed to create the individual lines. In contrast to this general trend, the N v lambda1240 equivalent width is nearly independent of continuum luminosity and remains nearly constant. The overall line behaviors are consistent with softer UV continuum shapes and perhaps increasing gas metallicity in more luminous active galactic nuclei.