Near-infrared spectroscopy of high-redshift active galactic nuclei. I. A metallicity-accretion rate relationship

We present new near-infrared spectroscopic measurements of the Hbeta region for a sample of 29 luminous high-redshift quasars. We have measured the width of Hbeta in those sources and added archival Hbeta width measurements to create a sample of 92 active galactic nuclei (AGNs) for which Hbeta width and rest-frame UV measurements of N V lambda1240 and C IV lambda1549 emission lines are available. Our sample spans 6 orders of magnitude in luminosity and includes 31 radio-loud AGNs. It also includes 10 narrow-line Seyfert 1 galaxies and one broad absorption line quasar. We find that metallicity, indicated by the N V/C IV line ratio, is primarily correlated with accretion rate, which is a function of luminosity and Hbeta line width. This may imply an intimate relation between starbursts, responsible for the metal enrichment of the nuclear gas, and AGN fueling, represented by the accretion rate. The correlation of metallicity with luminosity, or black hole (BH) mass, is weaker in contrast to recent results that were based on measurements of the width of C IV. We argue that using C IV as a proxy to Hbeta in estimating M-BH might be problematic and lead to spurious BH mass and accretion rate estimates in individual sources. We discuss the potential implications of our new result in the framework of the starburst-AGN connection and theories of BH growth.