Characterization of a sample of intermediate-type AGN. I. Spectroscopic
  properties and serendipitous discovery of new Dual AGN
  A sample of 10 nearby intermediate-type active galactic nuclei (AGN) drawn
from the Sloan Digital Sky Survey (SDSS-DR7) is presented. The aim of this work
is to provide estimations of the black hole 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 BPT diagnostic diagrams
we have carefully classified the objects as true intermediate-type AGN and
found that 80%$^{+7.2%}_{-17.3%}$ are composite AGN. The black hole mass
estimated for the sample is within 6.54$\pm$0.16\,$<$\,log\,$M_{\rm
BH}$\,$<$\,7.81$\pm$0.14. Profile analysis show that five objects
(\object{J120655.63+501737.1}, \object{J121607.08+504930.0},
\object{J141238.14+391836.5}, \object{J143031.18+524225.8} and
\object{J162952.88+242638.3}) have narrow double-peaked emission lines in both
the red (H$\alpha$, [\ion{N}{2}]$\lambda\lambda$6548,6583 and
[\ion{S}{2}]$\lambda\lambda$6716,6731) and the blue (H$\beta$ and
[\ion{O}{3}]$\lambda\lambda$4959,5007) region of the spectra, with velocity
differences ($\Delta V$) between the double peaks within 114\,$<\Delta
V\,<$\,256 km s$^{-1}$. Two of them, \object{J121607.08+504930.0} and
\object{J141238.14+391836.5} are candidates for dual AGN since their
double-peaked emission lines are dominated by AGN activity. In searches of dual
AGN; Type 1, Type 1I and intermediate-type AGN should be carefully separated,
due to the high serendipitous number of narrow double-peaked sources
(50%$\pm$14.4%) found in our sample.
