HST-COS Observations of AGNs. III. Spectral Constraints in the Lyman Continuum from Composite COS/G140L Data

Abstract

The rest-frame ultraviolet (UV) spectra of active galactic nuclei (AGNs) are important diagnostics of both accretion disk physics and their contribution to the metagalactic ionizing UV background. Though the mean AGN spectrum is well characterized with composite spectra at wavelengths greater than 912 Angstroms, the shorter-wavelength extreme-UV (EUV) remains poorly studied. In this third paper in a series on the spectra of AGNs, we combine 11 new spectra taken with the Cosmic Origins Spectrograph on the Hubble Space Telescope with archival spectra to characterize the typical EUV spectral slope of AGNs from ${\lambda }_{\mathrm{rest}}\sim 850\mathrm{Angstroms}$ down to ${\lambda }_{\mathrm{rest}}\sim 425\mathrm{Angstroms}$. Parameterizing this slope as a power law, we obtain $F_{\nu }\propto {\nu }^{-0.72\pm 0.26}$, but we also discuss the limitations and systematic uncertainties of this model. We identify broad emission features in this spectral region, including emission due to ions of O, Ne, Mg, and other species, and we limit the intrinsic HeI 504 Angstrom photoelectric absorption edge opacity to ${\tau }_{\mathrm{HeI}}<0.047$.

15 pages, 9 figures, 4 tables. Accepted to the Astrophysical Journal

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• AGNfitter: A Bayesian MCMC approach to fitting spectral energy distributions of AGN
• Iron Opacity Bump Changes the Stability and Structure of Accretion Disks in Active Galactic Nuclei
• Statistical Detection of the HeII Transverse Proximity Effect: Evidence for Sustained Quasar Activity for $>$25 Million Years
• Limits on the LyC signal from z~3 sources with secure redshift and HST coverage in the E-CDFS field
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• Spectral Evolution in High Redshift Quasars from the Final BOSS Sample
• Modeling the HeII Transverse Proximity Effect: Constraints on Quasar Lifetime and Obscuration
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• An Ultraviolet Survey of Low-Redshift Partial Lyman-Limit Systems with the HST Cosmic Origins Spectrograph
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• Spectral Energy Distribution Variations of Nearby Seyfert Galaxies During AGN Watch Monitoring Programs
• Simulating the epoch of Helium Reionization in photon-conserving semi-numerical code SCRIPT
• Measuring photo-ionization rate and mean free path of HeII ionizing photons at $2.5\le z\le 3.6$: Evidence for late and rapid HeII reionization Part-II
• Physical conditions and redshift evolution of optically thin C III absorbers: Low-z sample