A New Physical Picture for Active Galactic Nuclei Lacking Optical Emission Lines

In this work, we use similar to 500 low-redshift (z similar to 0.1) X-ray active galactic nuclei (AGNs) observed by XMM-Newton and the Sloan Digital Sky Survey (SDSS) to investigate the prevalence and nature of AGNs that apparently lack optical emission lines ("optically dull AGNs"). Although one quarter of spectra appear absorption-line dominated in visual assessment, line extraction with robust continuum subtraction from the MPA/JHU catalog reveals usable [O iii] measurements in 98% of the sample, allowing us to study [O iii]-underluminous AGNs together with more typical AGNs in the context of the L ([O III])-L (X) relation. We find that "optically dull AGNs" do not constitute a distinct population of AGNs. Instead, they are the [O iii]-underluminous tail of a single, unimodal L ([O III])-L (X) relation that has substantial scatter (0.6 dex). We find the degree to which an AGN is underluminous in [O iii] correlates with the specific star formation rate or D (4000) index of the host, which are both linked to the molecular gas fraction. Thus the emerging physical picture for the large scatter seems to involve the gas content of the narrow-line region. We find no significant role for previously proposed scenarios for the presence of optically dull AGNs, such as host dilution or dust obscuration. Despite occasionally weak lines in SDSS spectra, >80% of X-ray AGNs are identified as such with the Baldwin-Phillips-Terlevich diagram. More than 90% are classified as AGNs based only on [N ii]/H alpha, providing more complete AGN samples when [O iii] or H beta are weak. X-ray AGNs with LINER spectra obey essentially the same L ([O III])-L (X) relation as Seyfert 2s, suggesting their line emission is produced by AGN activity.