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

TL;DR

This study investigates luminous radio-quiet quasars with large C IV equivalent widths, revealing their enhanced UV and X-ray emissions, low Eddington ratios, and potential links between metallicity, radiation pressure, and X-ray suppression.

Contribution

It provides new insights into the spectral properties, metallicity indicators, and possible physical mechanisms of extreme low EV1 quasars, supported by observations and photoionization models.

Findings

Large C IV EW quasars have enhanced He II and Mg II lines.

Their X-ray emission exceeds expectations based on UV luminosity.

C IV/Mg II ratio correlates with X-ray excess and metallicity.

Abstract

We report results for a complete sample of ten luminous radio-quiet quasars with large C IV equivalent widths (EW > 150 A). 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 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 aox 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 sub-Solar 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.