The OI 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 OI $λ$8446, $λ$11287 and $λ$1304 in Active Galactic Nuclei. From the strength of the former two lines, contrary to the general belief, we found that in six out of seven galaxies, L-beta fluorescence is not the only mechanism responsible for the formation of these three lines. Because OI $λ$13165 is almost reduced to noise level, continuum fluorescence is ruled out as an additional excitation mechanism, but the presence of OI $λ$7774 in one of the objects suggests that collisional ionization may have an important role in the formation of OI $λ$8446. The usefulness of the OI lines as a reliable reddening indicator for the broad line region is discussed. The values of E(B-V) derived from the OI $λ1304/λ$8446 ratio agree with those obtained using other reddening indicators. The observations point toward a break in the one-to-one photon relation between OI $λ$8446 and OI $λ$1304, attributable to several destruction mechanisms that may affect the latter line.

26 pages, 5 figures. Accepted for publication in ApJ