REST-FRAME OPTICAL PROPERTIES of LUMINOUS 1.5 ^Z 3.5 QUASARS: The Hβ-[O III] REGION

We study the rest-frame optical properties of 74 luminous (Lbol 10 erg s = 46.2-48.2 -1), 1.5 ^z 3.5 broad-line quasars with near-IR (JHK) slit spectroscopy. Systemic redshifts based on the peak of the [O III] Λ5007 line reveal that redshift estimates from the rest-frame UV broad emission lines (mostly Mg II) are intrinsically uncertain by∼200 km s^-1(measurement errors accounted for). The overall full-width-at-half-maximum of the narrow [O III] line is ∼1000 km s^-1on average. A significant fraction of the total [O III] flux (∼40%) is in a blueshifted wing component with a median velocity offset of∼700 km s^-1, indicative of ionized outflows within a few kpc from the nucleus; we do not find evidence of significant [O III] flux beyond ∼10 kpc in our slit spectroscopy. The [O III] line is noticeably more asymmetric and weaker than that in typical less luminous low-z quasars. However, when matched in quasar continuum luminosity, low-z quasars have similar [O III] profiles and strengths as these high-z systems. Therefore the exceptionally large width and blueshifted wing, and the relatively weak strength of [O III] in high-z luminous quasars are mostly a luminosity effect rather than redshift evolution. The Hβ-[O III] region of these high-z quasars displays a similar spectral diversity and Eigenvector 1 correlations with anti-correlated [O III] and optical Fe II strengths, as seen in low-z quasars; but the average broad Hβ width is larger by 25% than typical low-z quasars, indicating more massive black holes in these high-z systems. These results highlight the mportance of understanding [O III] in the general context of quasar parameter space in order to understand quasar feedback in the form of [O III] outflows. The calibrated one-dimensional near-IR spectra are made publicly available, along with a composite spectrum.