Shedding New Light on Weak Emission-Line Quasars in the C IV-Hβ Parameter Space

Weak emission-line quasars (WLQs) are a subset of Type 1 quasars that exhibit extremely weak Lyα+N V lambda1240 and/or C IV lambda1549 emission lines. We investigate the relationship between emission-line properties and accretion rate for a sample of 230 'ordinary' Type 1 quasars and 17 WLQs at z < 0.5 and 1.5 < z < 3.5 that have rest-frame ultraviolet and optical spectral measurements. We apply a correction to the Hβ-based black-hole mass (M_BH) estimates of these quasars using the strength of the optical Fe II emission. We confirm previous findings that WLQs' M_BH values are overestimated by up to an order of magnitude using the traditional broad emission-line region size-luminosity relation. With this M_BH correction, we find a significant correlation between Hβ-based Eddington luminosity ratios and a combination of the rest-frame C IV equivalent width (EW) and C IV blueshift with respect to the systemic redshift. This correlation holds for both ordinary quasars and WLQs. Our results suggest that the two-dimensional C IV parameter space can serve as an indicator of accretion rate in all Type 1 quasars across a wide range of spectral properties.