The anti-correlations between the equivalent widths of emission lines and the continuum luminosity in AGNs , known as the Baldwin effect are well established for broad lines , but are less well studied for narrow lines .
In this paper we explore the Baldwin effect of narrow emission lines over a wide range of ionization levels and critical densities using a large sample of broad-line , radio-quiet AGNs taken from Sloan Digital Sky Survey ( SDSS ) Data Release 4 .
These type1 AGNs span three orders of magnitude in continuum luminosity .
We show that most narrow lines show a similar Baldwin effect slope of about -0.2 while the significant deviations of the slopes for [ N II ] \lambda 6583 , [ O II ] \lambda 3727 , [ Ne V ] \lambda 3425 , and the narrow component of H \alpha can be explained by the influence of metallicity , star-formation contamination and possibly by difference in the shape of the UV-optical continuum .
The slopes do not show any correlation with either the ionization potential or the critical density .
We show that a combination of 50 % variations in continuum near 5100Å and a log-normal distribution of observed luminosity can naturally reproduce a constant Baldwin effect slope of -0.2 for all narrow lines .
The variations of the continuum could be due to variability , intrinsic anisotropic emission , or an inclination effect .