IMPLICATIONS OF LYMAN-ALPHA EQUIVALENT WIDTHS IN ACTIVE GALACTIC NUCLEI

Variable Seyfert 1 nuclei exhibit a nonlinear response of Lyman alpha luminosity to changes in the continuum level, leading to a negative correlation between Lyalpha equivalent width and luminosity. We report the results of photoionization calculations that examine the sensitivity of Lyalpha emission to parameters describing the broadline clouds and incident radiation field. The luminosity dependence of the equivalent width of Lyalpha can be understood in terms of increased destruction of the line at higher ionization parameter (U; ratio of ionizing photon to hydrogen density). The observed slope of equivalent width versus continuum flux for NGC 5548 and Fairall 9 is consistent with an ionizing continuum shape that is independent of luminosity. Measurements of the slope of Lyalpha equivalent width versus continuum brightness limit the permitted combinations of U and cloud density characterizing the Lyalpha-emitting zone. We consider the case of the well-studied Seyfert 1 galaxy NGC 5548 and find good consistency with the observed properties of the Lyalpha zone for a cloud density of approximately 10(11) cm-3 and U near 0.2. Estimation of the cloud covering factor depends on the extrapolation of the extreme ultraviolet (EUV) continuum. EUV cutoffs at 20 and 60 eV correspond to a range of covering factors between 0.8 and 0.5, which is larger than typically assumed in the past for luminous Seyferts. Equivalent width observations of Fairall 9 show that its continuum cannot be as soft as suggested by previous line ratio analyses and imply that its EUV continuum persists to 20 eV or more before cutting off. Finally, we outline a method by which variability measurements can be combined with our calculations to measure the distances to active galactic nuclei, and hence the Hubble constant.