Spectral properties of AGN with very weak [O III] lines

Abstract

The spectral properties of a sample of 58 Active Galactic Nuclei (AGN) spectra, in which emission [O III] λλ4959, 5007 Å lines are weak or totally absent, are analyzed. In order to investigate the physical reason for the [O III] emission suppression, the spectral properties of the weak [O III] spectra sample are compared with the same properties of a sample of 269 spectra with the strong [O III] lines. The spectra are obtained from Sloan Digital Sky Survey (SDSS) Database. It is found that the objects with the weak or absent [O III] λλ4959, 5007 Å lines generally have the high continuum luminosities (log(λL5100) > 45), that they are very rare at smaller redshifts (z < 0.3) and that they usually have strong starburst influence. From the sample with weak or absent [O III] lines, two boundary subgroups may be distinguished: the subgroup with a strong Hβ narrow component and subgroup with a very weak or negligible Hβ narrow component. The physical causes for the [O III] lines suppressing are probably different in these two subgroups: the [O III] lines are absent in objects with strong narrow Hβ probably because of strong starburst (SB) activity, which produces high density of the gas, while in the objects with the negligible narrow Hβ, the reason for [O III] and narrow Hβ suppression may be a low covering factor.

Keywords

• galaxies: active
• galaxies: emission lines