Blueshifted [O III] emission: Indications of a dynamic narrow-line region

The [O III] lambda 5007 line is commonly used as an indicator of the systemic redshift of active galactic nuclei (AGNs). Also, recent studies have used the width of this emission line as a proxy for the stellar velocity dispersion in the host galaxy. This paper calls both of these assumptions into question by analyzing a sample of approximately 400 AGN spectra from the first data release of the Sloan Digital Sky Survey. These spectra show that the low-ionization forbidden lines ([O II], [N II], and [S II]) define a consistent redshift but that the peak of the [O III] line is blueshifted in approximately half of the AGNs with respect to that redshift. For the sample studied here, the average shift is 40 km s(-1), with the largest shift being more than 400 km s(-1). The magnitude of this shift is found to be correlated with a number of properties, including the width of the [O III] line and the Eddington ratio (L/L-Edd), derived from the luminosity and width of H beta.