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

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.

17 pages, 9 figures, accepted for publication in ApJ

References in corpus (17)

• The Zwicky Transient Facility: System Overview, Performance, and First Results
• A Catalog of Quasar Properties from SDSS DR7
• Some Aspects of Measurement Error in Linear Regression of Astronomical Data
• Biases in Virial Black Hole Masses: An SDSS Perspective
• Piecing Together the X-ray Background: Bolometric Corrections for Active Galactic Nuclei
• The X-ray Properties of the Most-Luminous Quasars from the Sloan Digital Sky Survey
• The diversity of quasars unified by accretion and orientation
• The Hard X-ray Spectrum as a Probe for Black-Hole Growth in Radio-Quiet Active Galactic Nuclei
• X-ray Insights into the Nature of PHL 1811 Analogs and Weak Emission-Line Quasars: Unification with a Geometrically Thick Accretion Disk?
• Are Optically-Selected Quasars Universally X-Ray Luminous? X-Ray/UV Relations in Sloan Digital Sky Survey Quasars
• The $L_{\mathrm{x}}$-$L_{\mathrm{uv}}$-$L_{\mathrm{radio}}$ relation and corona-disk-jet connection in optically selected radio-loud quasars
• The correlations between optical/UV broad lines and X-ray emission for a large sample of quasars
• Dust Reverberation Mapping in Distant Quasars from Optical and Mid-Infrared Imaging Surveys
• Black hole magnetosphere with small scale flux tubes--II. Stability and dynamics
• Sensitive Chandra coverage of a representative sample of weak-line quasars: revealing the full range of X-ray properties
• What controls the UV-to-X-ray continuum shape in quasars?
• Characterizing Quasar CIV Emission-line Measurements from Time-resolved Spectroscopy

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