Characterization of a sample of intermediate-type AGNs. I. spectroscopic properties and serendipitous discovery of new dual AGNs

A sample of 10 nearby intermediate-type active galactic nuclei (AGNs) drawn from the Sloan Digital Sky Survey is presented. The aim of this work is to provide estimations of the black hole (BH) mass for the sample galaxies from the dynamics of the broad-line region. For this purpose, a detailed spectroscopic analysis of the objects was done. Using Baldwin-Phillips-Terlevich diagnostic diagrams, we have carefully classified the objects as true intermediate-type AGNs and found that 80%^+7.2% _-17.3% are composite AGNs. The BH mass estimated for the sample is within 6.54 ± 0.16 < log M_BH < 7.81 ± 0.14. Profile analysis shows that five objects (J120655.63+501737.1, J121607.08+504930.0, J141238.14+391836.5, J143031.18+524225.8, and J162952.88+242638.3) have narrow double-peaked emission lines in both the red (Hα, [N II] λλ6548,6583 and [S II] λλ6716, 6731) and the blue (Hβ and [O III] λλ4959, 5007) regions of the spectra, with velocity differences (ΔV) between the double peaks within 114 km s^-1 < ΔV < 256 km s^-1. Two of them, J121607.08+504930.0 and J141238.14+391836.5, are candidates for dual AGNs since their double-peaked emission lines are dominated by AGN activity. In searches of dual AGNs, type 1, type II, and intermediate-type AGNs should be carefully separated, due to the high serendipitous number of narrow double-peaked sources (50% ± 14.4%) found in our sample.