The effect of intrinsic ultraviolet absorbers on the ionizing continuum and narrow emission line ratios in Seyfert galaxies

We explore the effects of UV-absorbing material on the shape of the EUV con tinuum radiation emitted by the active galactic nucleus and on the relative strengths of emission lines, formed in the narrow-line regions of Seyfert galaxies, excited by this continuum. Within a sample of Seyfert 1.5 galaxie s, objects with flatter soft X-ray slopes tend to have lower values of He I I lambda 4686/H beta, which implies a correlation between the observed spec tral energy distribution of the ionizing continuum and the narrow emission line strengths. Objects with the flattest soft X-ray continua tend to posse ss high-column density UV absorption, and it is plausible that the differen ces in narrow emission line ratios among these galaxies are an indication o f the effects of absorbing material internal to the narrow-line region, rat her than intrinsic differences in continuum shape. We have generated a set of photoionization models to examine the effect of a range of UV absorbers on the ionizing continuum and, hence, the resulting conditions in a typical narrow-line cloud. Our results indicate that a low-ionization UV absorber with large covering factor will indeed produce the combination of narrow-li ne ratios and soft X-ray spectral characteristics observed in several Seyfe rt 1.5 galaxies. Our results also suggest that low-ionization UV absorption may be more common than currently believed.