Active Galactic Nucleus Emission‐Line Properties Versus the Eddington Ratio

Abstract

We analyze UV spectra for a large sample of 578 type 1 active galactic nuclei and derive Eddington ratios, L/LEdd, from the bolometric luminosities and emission-line widths for each object in the sample. The sample spans 5 orders of magnitude in supermassive black hole (SMBH) mass, 7 orders of magnitude in luminosity, and a redshift range 0 ≤ z ≤ 5. We include a sample of 26 low-redshift narrow-line Seyfert 1 galaxies (NLS1's) for comparative analysis. The NLS1's have slightly larger than average L/LEdd ratios (and smaller SMBH masses) for their luminosities, but those L/LEdd values are still substantially below the average for luminous quasars. A large fraction (27%) of the objects overall have L/LEdd > 1, which might be explained by nonspherically symmetric accretion. We find no trend between L/LEdd and either redshift or SMBH mass. Composite spectra sorted by L/LEdd show an unusual emission-line behavior: nearly constant peak heights and decreasing FWHMs with increasing L/LEdd. This is in marked contrast to the emission-line behaviors with luminosity, SMBH mass, and FWHM(C IV), which clearly show trends analogous to the Baldwin effect: decreasing line peaks and equivalent widths with increasing luminosity, SMBH mass, and FWHM. The origins of the unusual behavior with L/LEdd are not understood, but one implication is that metallicity estimates based on emission-line ratios involving nitrogen show no trend with L/LEdd in the composite spectra created from different ranges in L/LEdd. The NLS1 composite, however, shows a slightly high metallicity for its SMBH mass and luminosity. Our earlier work suggests that host galaxy mass, correlated with SMBH mass and AGN luminosity, is the fundamental parameter affecting broad emission line region metallicities. Some secondary effect, not related to L/LEdd, must be enhancing the metallicities in NLS1's.