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

Abstract

We present near-infrared spectroscopy of ten weak emission-line quasars (WLQs) at redshifts of $z\sim 2$, 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 $\hat{I}»1549$ line), and they display remarkable X-ray properties. We derive H${\hat{I}}^2$-based single-epoch virial black-hole masses (median value $1.7\times 10^9{\mathrm{M}}_{\odot }$) and Eddington ratios (median value $0.5)$ for our sources. We confirm the previous finding that WLQ H${\hat{I}}^2$ 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] $\hat{I}»5007$ 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 $4900km{\mathrm{s}}^{-1}$) in one object.

19 pages, 8 figures, accepted for publication in ApJ