Type 1 Active Galactic Nucleus Fraction in SDSS/FIRST Survey
  In the unification scheme, narrow-lined (type 2) active galactic nuclei (AGN)
are intrinsically similar to broad-lined (type 1) AGN with the exception that
the line of sight to the broad emission line region and accretion disk is
blocked by a dusty torus. The fraction of type 1 AGN measures the average
covering factor of the torus. In this paper, we explore the dependence of this
fraction on nuclear properties for a sample of low redshift (z <0.35) radio
strong (P_{1.4GHz} >10^{23}W/Hz) AGN selected by matching the spectroscopic
catalog of Sloan Digital Sky Survey and the radio source catalog of Faint Image
of Radio Sky at Twenty cm. After correcting for several selection effects, we
find that : (1) type 1 fraction $f_1$ keeps at a constant of ~20 per cent in
the [O III] 5007 luminosity range of 40.7< log(L_{[O III]}/ erg/s) <43.5 . This
result is significantly different from previous studies, and the difference can
be explained by extinction correction and different treatment of selection
effects. (2) $f_1$ rises with black hole mass from ~20 per cent (M_bh below
10^8 Msun) to ~30 per cent (M_bh above that). This coincides with the decrease
of the fraction of highly-inclined disk galaxies with black hole mass, implying
a population of Seyfert galaxies seen as type 2 due to galaxy-scale obscuration
in disk when the host galaxy type transfer from bulge-dominant to
disk-dominant.
  (3) $f_1$ is independent of the Eddington ratio for its value between 0.01
and 1; (4) $f_1$ ascends from 15 per cent to 30 per cent in the radio power
range of 23< log(P_{1.4GHz}/ W/Hz) <24, then remain a constant at ~30 per cent
up to 10^{26} W/Hz.
