Cosmic evolution of mass accretion rate and metallicity in active galactic nuclei

We present line and continuum measurements for 9818 SDSS type 1 active galactic nuclei (AGNs) with z ≤ 0.75. The data are used to study the four-dimensional space of black hole mass, normalized accretion rate (L/Z/Edd), metallicity, and redshift. The main results are (1) L/L_Edd is smaller for larger mass black holes at all redshifts. (2) For a given black hole mass, L/L_Edd ∞ z^γ or (1 + z)^δ, where the slope γ increases with black hole mass. The mean slope is similar to the star formation rate slope over the same redshift interval. (3) The Fe II/Hβ line ratio is significantly correlated with L/L_Edd It also shows a weaker negative dependence on redshift. Combined with the known dependence of metallicity on accretion rate, we suggest that the Fe II/Hβ line ratio is a metallicity indicator. (4) Given the measured accretion rates, the growth times of most AGNs exceed the age of the universe. This suggests past episodes of faster growth for all those sources. Combined with the Fe II/Hβ result, we conclude that the broad emission line metallicity goes through cycles and is not a monotonously decreasing function of redshift. (5) FWHM([0 _III γ5007) is a poor proxy for σ*, especially for high LIL_Edd. (6) We define a group of narrow-line type 1 AGNs (NLAGN1s) by their luminosity- (or mass-) dependent Hβ line width. Such objects have L/L_Edd≥ 0.25, and they comprise 8% of the type 1 population. Other interesting results include negative Baldwin relationships for EW(Hβ) and EW(Fe II) and a relative increase of the red part of the Hβ line with luminosity.