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      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

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          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 \(\thickapprox 2.72 \pm 0.04\), consistent with the 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 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 underestimated by an order of magnitude. This will be the main cause of the discrepancies between measured accretion disc sizes and the predictions of simple accretion disc theory. The greater luminosity of AGNs also resolves the photon underproduction problem for the intergalactic medium. The total mass in lower mass supermassive black holes must be greater than hitherto estimated.

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          Journal
          1512.09291

          Galaxy astrophysics
          Galaxy astrophysics

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