A universal average spectral energy distribution for quasars from optical to extreme ultraviolet

Abstract

The well-known anti-correlation between the optical/ultraviolet (UV) emission line equivalent widths of active galactic nuclei and the continuum luminosity (the so-called Baldwin effect) is a long-standing puzzle. One common hypothesis is that more luminous sources have softer spectral energy distribution (SED) in the extreme UV (EUV), as revealed by some observational studies. In this work we revisit this issue through cross-matching SDSS quasars with GALEX far-UV/near-UV catalogs and correcting the effect of a severe observational bias of significant UV detection incompleteness, i.e., the more luminous in observed-frame optical, the more likely detected in observed-frame UV. We find that, for GALEX detected quasars at 1.8 < z < 2.2, the rest-frame mean UV SED (~ 500 -- 3000 Angstrom) bewilderingly shows no luminosity dependence at log(νL_ν(2200 Angstrom)) > 45 (up to 47.3), contrary to the standard thin disc model predictions and the observed Baldwin effect in this luminosity range. Probably, the universal mean UV SED is the result of a local atomic-originated process, and in fainter quasars stronger disk turbulence launching more clouds is the main origin of the Baldwin effect. After correcting for the absorption of the intergalactic medium, a rest-frame intrinsic mean EUV SED is derived from a sub-sample of bright quasars and is found to be much redder in the EUV than all previous quasar composite spectra, highlighting the significance of properly accounting for the sample incompleteness. Interestingly, the global consistence between our extremely red mean EUV SED and the line-driven wind model again supports an origin of a local physical process.

27 pages, 15 figures, author's initial version submitted to Nature Astronomy

References in corpus (11)

• Modeling the Time Variability of SDSS Stripe 82 Quasars as a Damped Random Walk
• The first ultraviolet quasar stacked spectrum at z=2.4 from WFC3
• Quasar Accretion Disks Are Strongly Inhomogeneous
• Mean Spectral Energy Distributions and Bolometric Corrections for Luminous Quasars
• The characteristic blue spectra of accretion disks in quasars as uncovered in the infrared
• HST-COS Observations of AGN. II. Extended Survey of Ultraviolet Composite Spectra from 159 Active Galactic Nuclei
• The UV Continuum of Quasars: Models and SDSS Spectral Slopes
• Eddington ratio governs the equivalent width of MgII emission line in active galactic nuclei
• Variation of ionizing continuum: the main driver of Broad Absorption Line Variability
• Opacity Driven Convection and Variability in Accretion Disks around Supermassive Black Holes
• The UV Properties of SDSS Selected Quasars

Cited by in corpus (1)

• The Host Galaxy Fluxes of Active Galaxy Nuclei Are Generally Overestimated by the Flux Variation Gradient Method