Near-Infrared Spectroscopy of High-Redshift Active Galactic Nuclei. II. Disappearing Narrow-Line Regions and the Role of Accretion

We present new near-infrared spectroscopic measurements for 29 luminous high-redshift active galactic nuclei (AGNs) and use the data to discuss the size and other properties of the narrow-line regions (NLRs) in those sources. The high-resolution spectra have been used to carefully model the Fe II blends and to provide reliable [O III] λ5007, Fe II, and Hβ measurements. We find that about two-thirds of all very high luminosity sources show strong [O III] λ5007 lines, while the remaining objects show no or very weak such lines. While weak [O III] λ5007 emitters are also found among lower luminosity AGNs, we argue that the implications for very high luminosity objects are different. In particular, we suggest that the averaging of these two populations in other works gave rise to claims of a Baldwin relationship in [O III] λ5007, which is not confirmed by our data. We also argue that earlier proposed relations of the type R_NLR ∝ L, where R_NLR is the radius of the NLR, are theoretically sound, yet they must break down for R_NLR exceeding a few kiloparsecs. This suggests that the NLR properties in high-luminosity sources are very different from those observed in nearby AGNs. In particular, we suggest that some sources lost their very large, dynamically unbound NLR, while others are in a phase of violent star-forming events that produce a large quantity of high-density gas in the central kiloparsec. This gas is ionized and excited by the central radiation source, and its spectroscopic properties may be different from those observed in nearby, lower luminosity NLRs. We also discuss the dependence of EW (Hβ) and Fe II/Hβ on luminosity, black hole mass, and accretion rate for a large sample of AGNs. The strongest dependence of the two quantities is on the accretion rate, and the Fe II/Hβ correlation is probably due to the EW (Hβ) dependence on accretion rate. We show the most extreme values measured so far of Fe II/Hβ and address its correlation with EW ([O III] λ5007).