Rest-Frame Optical Spectroscopy of Ten z ~ 2 Weak Emission-Line Quasars

We present near-infrared spectroscopy of ten weak emission-line quasars (WLQs) at redshifts of z~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 λ 1549 line), and they display remarkable X-ray properties. We derive Hβ-based single-epoch virial black-hole masses (median value 1.7 × 10⁹ M_) and Eddington ratios (median value $0.5)$ for our sources. We confirm the previous finding that WLQ Hβ 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] λ 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 ≈ 500 and 4900 km s⁻¹) in one object.