Active galactic nuclei in the sloan digital sky survey. II. Emission-line luminosity function

The emission-line luminosity function of active galactic nuclei (AGNs) is measured from about 3000 AGNs included in the main galaxy sample of the Sloan Digital Sky Survey within a redshift range of 0 < z < 0.15. The Hα and [O III] λ5007 luminosity functions for Seyfert galaxies cover a luminosity range of 10 ⁵-10 ⁹ L _⊙ in Hα, and the shapes are well fitted by broken power laws, without a turnover at fainter nuclear luminosities. Assuming a universal conversion from emission-line strength to continuum luminosity, the inferred 5-band magnitude luminosity function is comparable to both the AGN luminosity function of previous studies and the low-redshift quasar luminosity function derived from the Two-Degree Field redshift survey. The inferred AGN number density is approximately one-fifth of all galaxies, and about 6 × 10 ^-3 of the total light of galaxies in the r band comes from nuclear activity. The numbers of Seyfert 1 and Seyfert 2 galaxies are comparable at low luminosity, while at high luminosity, Seyfert 1 galaxies outnumber Seyfert 2 galaxies by a factor of 2-4. In making the luminosity function measurements, we assume that the nuclear luminosity is independent of the host galaxy luminosity, an assumption we test a posteriori and show to be consistent with the data. Given the relationship between black hole mass and host galaxy bulge luminosity, the lack of correlation between nuclear and host luminosity suggests that the main variable that determines the AGN luminosity is the Eddington ratio, not the black hole mass. This appears to be different from luminous quasars, which are most likely to be shining near the Eddington limit.