The relationship between [O III]λ5007 Å equivalent width and obscuration in active galactic nuclei

Abstract

In this paper we study the relationship between the equivalent width (EW) of the [O III]λ5007 Å narrow emission line in active galactic nuclei (AGN) and the level of obscuration. To this end, we combine the results of a systematic spectral analysis, both in the optical and in the X-rays, on a statistically complete sample of ∼170 X-ray selected AGN from the XMM-Newton Bright Serendipitous Source (XBS) sample. We find that the observed large range of [O III]λ5007 Å EWs observed in the sample (from a few Å up to 500 Å) is well explained as a combination of an intrinsic spread, probably due to the large range of covering factors of the narrow line region, and the effect of absorption. The intrinsic spread is dominant for EW below 40-50 Å while absorption brings the values of EW up to ∼100-150 Å for moderate levels of absorption (A_V∼ 0.5-2 mag) or up to ∼500 Å for A_V > 2 mag. In this picture, the absorption has a significant impact on the observed EW also in type 1 AGN. Using numerical simulations we find that this model is able to reproduce the [O III]λ5007 Å EW distribution observed in the XBS sample and correctly predicts the shape of the EW distribution observed in the optically selected sample of quasi-stellar object taken from the Sloan Digital Sky Survey.