The Relationship of Hard X‐Ray and Optical Line Emission in Low‐Redshift Active Galactic Nuclei

Abstract

In this paper we assess the relationship of the population of active galactic nuclei (AGNs) selected by hard X-rays to the traditional population of AGNs with strong optical emission lines. First, we study the emission-line properties of a new hard-X-ray-selected sample of 47 local AGNs (classified optically as Type 1 and 2 AGNs). We find that the hard X-ray (3-20 keV) and [O III] λ5007 optical emission-line luminosities are well-correlated over a range of about 4 orders of magnitude in luminosity (mean luminosity ratio 2.15 dex with a standard deviation of σ = 0.51 dex). Second, we study the hard X-ray properties of a sample of 55 local AGNs selected from the literature on the basis of the flux in the [O III] line. The correlation between the hard X-ray (2-10 keV) and [O III] luminosity for the Type 1 AGNs is consistent with what is seen in the hard-X-ray-selected sample. However, the Type 2 AGNs have a much larger range in the luminosity ratio, and many are very weak in hard X-rays (as expected for heavily absorbed AGNs). We then compare the hard X-ray (3-20 keV) and [O III] luminosity functions of AGNs in the local universe. These have similar faint-end slopes, with a luminosity ratio of 1.60 dex (0.55 dex smaller than the mean value for individual hard-X-ray-selected AGNs). We conclude that at low redshift, selection by narrow optical emission lines will recover most AGNs selected by hard X-rays (with the exception of BL Lac objects). However, selection by hard X-rays misses a significant fraction of the local AGN population with strong emission lines.