The Nature of Luminous Quasars with Very Large C iv Equivalent Widths

We report results for a complete sample of 10 luminous radio-quiet quasars with large C iv equivalent widths (EW ≥ 150 Å). For 8/10 we performed Chandra snapshot observations. We find that, in addition to the enhanced C iv line EW, their He ii and Mg ii lines are enhanced, but the C iii] line is not. Their X-ray emission is substantially stronger than expected from their ultraviolet luminosity. Additionally, these large C iv EW quasars show small C iv blueshifts and possibly low Eddington ratios, suggesting that they are “extreme low Eigenvector 1 (EV1)” quasars. The mean excess He ii EW is well matched by radiation pressure compression (RPC) photoionization models, with the harder α _ox ionizing spectrum. However, these results do not reproduce well the enhancement pattern of the C iv, Mg ii, and C iii] EWs, or the observed high C iv/Mg ii ratio. RPC calculations indicate that the C iv/Mg ii line ratio is an effective metallicity indicator, and models with subsolar-metallicity gas and a hard ionizing continuum reproduce well the enhancement pattern of all four ultraviolet lines. We find that the C iv/Mg ii line ratio in quasars is generally correlated with the excess X-ray emission. Extremely high EV1 quasars are characterized by high metallicity and suppressed X-ray emission. The underlying mechanism relating gas metallicity and X-ray emission is not clear but may be related to radiation-pressure-driven disk winds, which are enhanced at high metallicity, and consequent mass loading reducing coronal X-ray emission.