Rest-frame Optical Properties of Luminous 1.5<z<3.5 Quasars: the Hbeta-[OIII] Region

TL;DR

This study analyzes the optical properties of high-redshift luminous quasars, revealing that their [OIII] emission features are primarily luminosity-driven and comparable to low-redshift quasars when matched in luminosity, with implications for quasar feedback.

Contribution

It provides the first detailed rest-frame optical analysis of 74 luminous quasars at 1.5<z<3.5, showing [OIII] properties are luminosity-dependent rather than redshift-dependent.

Findings

[OIII] blueshifted wing indicates ionized outflows.

High-z quasars have larger broad Hbeta widths, implying more massive black holes.

[OIII] properties are similar to low-z quasars when matched in luminosity.

Abstract

We study the rest-frame optical properties of 74 luminous (L_bol=10^46.2-48.2 erg/s), 1.5<z<3.5 broad-line quasars with near-IR (JHK) slit spectroscopy. Systemic redshifts based on the peak of the [OIII]5007 line reveal that redshift estimates from the rest-frame UV broad emission lines (mostly MgII) are intrinsically uncertain by ~ 200 km/s (measurement errors accounted for). The overall full-width-at-half-maximum of the narrow [OIII] line is ~ 1000 km/s on average. A significant fraction of the total [OIII] flux (~ 40%) is in a blueshifted wing component with a median velocity offset of ~ 700 km/s, indicative of ionized outflows within a few kpc from the nucleus; we do not find evidence of significant [OIII] flux beyond ~ 10 kpc in our slit spectroscopy. The [OIII] 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 [OIII] profiles and strengths as these high-z systems. Therefore the exceptionally large width and blueshifted wing, and the relatively weak strength of [OIII] in high-z luminous quasars are mostly a luminosity effect rather than redshift evolution. The Hbeta-[OIII] region of these high-z quasars displays a similar spectral diversity and Eigenvector 1 correlations with anti-correlated [OIII] and optical FeII strengths, as seen in low-z quasars; but the average broad Hbeta width is larger by 25% than typical low-z quasars, indicating more massive black holes in these high-z systems. These results highlight the importance of understanding [OIII] emission in the general context of quasar parameter space in order to understand quasar feedback in the form of [OIII] outflows. The calibrated one-dimensional near-IR spectra are made publicly available, along with a composite spectrum.