Hu, C (reprint author), Chinese Acad Sci, Natl Astron Observ, Beijing 100012, Peoples R China.
Broad Fe II emission is a prominent feature of the optical and ultraviolet spectra of quasars. We report on a systematical investigation of optical Fe II emission in a large sample of 4037 z < 0.8 quasars selected from the Sloan Digital Sky Survey. We have developed and tested a detailed line-fitting technique, taking into account the complex continuum and narrow and broad emission-line spectra. Our primary goal is to quantify the velocity broadening and velocity shift of the Fe II spectrum in order to constrain the location of the Fe II-emitting region and its relation to the broad-line region. We find that the majority of quasars show Fe II emission that is redshifted, typically by similar to 400 km s(-1), but up to 2000 kms (-1), with respect to the systemic velocity of the narrow-line region or of the conventional broad-line region as traced by the H beta line. Moreover, the line width of Fe II is significantly narrower than that of the broad component of H beta. We show that the magnitude of the Fe II redshift correlates inversely with the Eddington ratio, and that there is a tendency for sources with redshifted Fe II emission to show red asymmetry in the H beta line. These characteristics strongly suggest that Fe II originates from a location different from, and most likely exterior to, the region that produces most of H beta. The Fe II-emitting zone traces a portion of the broad-line region of intermediate velocities whose dynamics may be dominated by infall.