The relationship of hard X-ray and optical line emission in low-redshift active galactic nuclei

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] lambda 5007 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 sigma 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.