Type 1 active galactic nucleus fraction in the SDSS/FIRST survey

In the unification scheme, narrow-lined (type 2) active galactic nuclei (AGNs) are intrinsically similar to broad-lined (type 1) AGNs, with the exception that the line of sight to the broad emission-line region and accretion disc is blocked by a dusty torus. The fraction of type 1 AGNs 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(1.4 GHz) >= 1023 W Hz-1) AGNs, selected by matching the spectroscopic catalogue of the Sloan Digital Sky Survey and the radio source catalogue of the Faint Image of Radio Sky at Twenty cm survey. After correcting for several selection effects, we find the following. (i) The type 1 fraction f(1) remains at a constant of similar to 20 per cent in the [O iii] luminosity range of 40.7 < log(L([O iii])/erg s-1) < 43.5. This result is significantly different from those of previous studies. The difference can be explained by extinction correction and a different treatment of selection effects. (ii) f(1) increases with the black hole mass from similar to 20 per cent below 108 M(circle dot) to 30 per cent above this. This coincides with the decrease of the fraction of highly inclined disc galaxies with black hole mass, implying a population of Seyfert galaxies seen as type 2 as a result of galaxy-scale obscuration in the disc when the host galaxy type transfers from bulge-dominant to disc-dominant. (iii) f(1) is independent of the Eddington ratio for its value between 0.01 and 1. (iv) f(1) increases from 15 to 30 per cent in the radio power range of 23 < log(P(1.4 GHz)/W Hz-1) < 24, then remains constant at similar to 30 per cent until 1026 W Hz-1.