Rest-Frame Optical Spectroscopy of Ten z $\sim$ 2 Weak Emission-Line Quasars

TL;DR

This study presents near-infrared spectra of ten z~2 weak emission-line quasars, revealing their black hole masses, accretion rates, and spectral features, supporting a super-Eddington accretion model with obscured high-energy radiation.

Contribution

First near-infrared spectroscopic analysis of ten WLQs at z~2, providing insights into their black hole properties and spectral characteristics, and supporting the super-Eddington accretion scenario.

Findings

WLQs have median black hole mass of 1.7 x 10^9 M_sun.

WLQs exhibit weak/absent [O III] emission and strong Fe II emission.

Evidence of extreme [O III] outflows in one object.

Abstract

We present near-infrared spectroscopy of ten weak emission-line quasars (WLQs) at redshifts of $z\sim2$, obtained with the Palomar 200-inch Hale Telescope. WLQs are an exceptional population of type 1 quasars that exhibit weak or no broad emission lines in the ultraviolet (e.g., the C IV $\lambda 1549$ line), and they display remarkable X-ray properties. We derive H$\beta$-based single-epoch virial black-hole masses (median value $\rm 1.7 \times 10^{9} M_{\odot}$) and Eddington ratios (median value $0.5)$ for our sources. We confirm the previous finding that WLQ H$\beta$ lines, as a major low-ionization line, are not significantly weak compared to typical quasars. The most prominent feature of the WLQ optical spectra is the universally weak/absent [O III] $\lambda 5007$ emission. They also display stronger optical Fe II emission than typical quasars. Our results favor the super-Eddington accretion scenario for WLQs, where the weak lines are a result of a soft ionizing continuum; the geometrically thick inner accretion disk and/or its associated outflow is responsible for obscuring the nuclear high-energy radiation and producing the soft ionizing continuum. We also report candidate extreme [O III] outflows (blueshifts of $\approx 500$ and $\rm 4900 km s^{-1}$) in one object.