The case for cases B and C: intrinsic hydrogen line ratios of the broad-line region of active galactic nuclei, reddenings, and accretion disc sizes

Abstract

Low-redshift active galactic nuclei (AGNs) with extremely blue optical spectral indices are shown to have a mean, velocity-averaged, broad-line H$\alpha$/H$\beta$ ratio of $\approx 2.72\pm 0.04$, consistent with a Baker-Menzel Case B value. Comparison of a wide range of properties of the very bluest AGNs with those of a luminosity-matched subset of the Dong et al. blue AGN sample indicates that the only difference is the internal reddening. Ultraviolet fluxes are brighter for the bluest AGNs by an amount consistent with the flat AGN reddening curve of Gaskell et al.(2004). The lack of a significant difference in the GALEX (FUV--NUV) colour index strongly rules out a steep SMC-like reddening curve and also argues against an intrinsically harder spectrum for the bluest AGNs. For very blue AGNs the Ly$\alpha$/H$\beta$ ratio is also consistent with being the Case B value. The Case B ratios provide strong support for the self-shielded broad-line model of Gaskell, Klimek & Nazarova. It is proposed that the greatly enhanced Ly$\alpha$/H$\beta$ ratio at very high velocities is a consequence of continuum fluorescence in the Lyman lines (Case C). Reddenings of AGNs mean that the far-UV luminosity is often underestimated by up to an order of magnitude. This is a major factor causing the discrepancies between measured accretion disc sizes and the predictions of simple accretion disc theory. Dust covering fractions for most AGNs are lower than has been estimated. The total mass in lower mass supermassive black holes must be greater than hitherto estimated.

15 pages, 6 figures. Accepted for publication in Monthly Notices of the Royal Astronomical Society

References in corpus (2)

• NGC 5548: The AGN Energy Budget Problem and the Geometry of the Broad-Line Region and Torus
• Attenuation from the optical to the extreme ultraviolet by dust associated with broad absorption line quasars: the driving force for outflows

Cited by in corpus (44)

• Quasar Accretion Disk Sizes From Continuum Reverberation Mapping From the Dark Energy Survey
• Discovery of Highly Blueshifted Broad Balmer and Metastable Helium Absorption Lines in a Tidal Disruption Event
• Quasar Accretion Disk Sizes from Continuum Reverberation Mapping in the DES Standard Star Fields
• Constraints on the Broad Line Region Properties and Extinction in Local Seyferts
• Accretion Disk Sizes from Continuum Reverberation Mapping of AGN Selected from the ZTF Survey
• AGN Continuum Reverberation Mapping Based on Zwicky Transient Facility Light Curves
• AT 2017gbl: a dust obscured TDE candidate in a luminous infrared galaxy
• Space Telescope and Optical Reverberation Mapping Project. X. Understanding the Absorption-Line Holiday in NGC 5548
• Partial dust obscuration in active galactic nuclei as a cause of broad-line profile and lag variability, and apparent accretion disc inhomogeneities
• AT 2019avd: A novel addition to the diverse population of nuclear transients
• Accretion Disk Size Measurements of Active Galactic Nuclei Monitored by the Zwicky Transient Facility
• Gamma-ray opacity of the anisotropic stratified broad-line regions in blazars
• The curtain remains open: NGC 2617 continues in a high state
• Reddening of the BLR and NLR in AGN From a Systematic Analysis of Balmer Decrement
• Modeling accretion disk emission with generalized temperature profile and its effect on AGN spectral energy distribution
• The post-maximum behaviour of the changing-look Seyfert galaxy NGC 1566
• The relationship between X-ray and optical absorbers in active galactic nuclei
• Optical continuum photometric reverberation mapping of the Seyfert-1 galaxy Mrk509
• Chemical abundances in Seyfert galaxies -IX. Helium abundance estimates
• Transient obscuration event captured in NGC 3227. I. Continuum model for the broadband spectral energy distribution
• Modeling photometric reverberation mapping data for the next generation of big data surveys. Quasar accretion disks sizes with the LSST
• Intermediate-line Emission in AGNs: The Effect of Prescription of the Gas Density
• Fantastic Fits with fantasy of Active Galactic Nuclei Spectra -- Exploring the Fe II emission near the H$\alpha$ line
• Estimating reddening of the continuum and broad-line region of active galactic nuclei: the mean reddening of NGC 5548 and the size of the accretion disc
• The Paschen Jump as a Diagnostic of the Diffuse Nebular Continuum Emission in Active Galactic Nuclei
• The SOUX AGN Sample: SDSS-XMM-Newton Optical, Ultraviolet and X-ray selected active galactic nuclei spanning a wide range of parameter space -- Sample definition
• A Mid-infrared Flare in the Active Galaxy MCG-02-04-026: Dust Echo of a Nuclear Transient Event
• The 'Big Dipper': The nature of the extreme variability of the AGN SDSS J2232-0806
• Photoionization Modelling of the Giant Broad-Line Region in NGC 3998
• AstroSat/UVIT observations of IC4329A: Constraining the accretion disc inner radius
• Measurement of AGN dust extinction based on the near-infrared flux variability of WISE data
• Changing looks of the nucleus of the Seyfert galaxy NGC 1566 compared with other changing-look AGNs
• Far-Ultraviolet Spectroscopy Of Active Galactic Nuclei With AstroSat/UVIT
• Temperature Fluctuations in Quasar Accretion Discs from Spectroscopic Monitoring Data
• Efficient Analysis Routines for Single and Double Peaked Type 2 AGN Spectra
• The optical vs. mid-infrared spectral properties of 82 Type 1 AGNs: coevolution of AGN and starburst
• Spectral variability studies in Active Galactic Nuclei: Exploring continuum and emission line regions in the age of LSST and JWST
• Reddening and the Shape of the Variable Component of the Continua of Active Galactic Nuclei From the Optical to the Far Ultraviolet. I
• Updated Picture of the Active Galactic Nuclei with Dusty/Dust-free Gas Structures and Effects of the Radiation Pressure
• The variability of the broad-line Balmer decrement for quasars from the Sloan Digital Sky Survey Reverberation Mapping
• Influence of Comptonization region over the ambiance of accretion disc in Active Galactic Nucleus
• Fading Light, Fierce Winds: JWST Snapshot of a Sub-Eddington Quasar at Cosmic Dawn
• The Southern Hemisphere Narrow-Line Seyfert 1 Infrared Survey
• B2 0003+38A: a classical flat-spectrum radio quasar hosted by a rotation-dominated galaxy with a peculiar massive outflow