On the Correlation between X-Ray Flux and Hydrogen Line Intensities in Quasars and Active Galactic Nuclei

Hydrogen line intensities in quasars and active galactic nuclei are calculated as a function of the UV/X-ray luminosity ratio. The dependence of Ly-alpha/H-alpha (or Ly-alpha/H-beta) on L(uv)/Lx is found to be weak and not easily discernible in a large data sample if variations in the cloud column and nucleon density are large. Two causes account for the weak dependence. At high X-ray luminosities, Balmer continuum emission is the dominant coolant in the extended ionized zone (where X-ray heating occurs) of a broad-line cloud, and the Balmer line intensities increase very slowly with increasing X-ray flux. At low X-ray luminosities, both ionization from excited states of N I and Ly-beta continuum resonance fluorescence enable the gas to absorb continuum photons between about 11.5 and 13.6 eV, and the consequent heating, now relatively important, enhances H-alpha and H-beta emissions.