Theoretical hydrogen-line ratios for the narrow-line regions of active galactic nuclei

Abstract

The authors present calculations of the relative strengths of Lα, Hα, Hβ, Hγ, and Pα for low-density gas clouds photoionized by a continuum such as might be found in active galactic nuclei. They confirm the finding of Halpern (1982) that the Hα/Hβ ratio is insensitive to the ionization parameter. The ratio also depends only weakly on the electron density. Hα/Hβ is mostly a function of metallicity and the shape of the UV to X-ray continuum. It is recommended that Hα/Hβ should be "reinstated" as an indicator of reddening for the narrow-line regions of active galaxies, but with an assumed intrinsic value of about 3.0, as proposed by Halpern, rather than the previous case B value of 2.85.