The O [iota] line emission in active galactic nuclei revisited

Abstract

UV, visible, and near-infrared spectroscopy is used to study the transitions of neutral oxygen leading to the emission of broad O i λ8446, λ11287, and λ1304 in active galactic nuclei. From the strength of the former two lines, contrary to the general belief, we found that in six of seven galaxies, Lyβ fluorescence is not the only mechanism responsible for the formation of these three lines. Because O i λ13165 is almost reduced to noise level, continuum fluorescence is ruled out as an additional excitation mechanism, but the presence of O i λ7774 in one of the objects suggests that collisional ionization may have an important role in the formation of O i λ8446. The usefulness of the O i lines as a reliable reddening indicator for the broad-line region is discussed. The values of E(B−V) derived from the λ1304/ λ8446 ratio agree with those obtained using other reddening indicators. The observations point toward a break in the one-to-one photon relation between λ8446 and λ1304, attributable to several destruction mechanisms that may affect the latter line.