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 continuum 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 galaxies, objects with flatter soft X-ray slopes tend to have lower values of He II lambda 4686/H beta, which implies a correlation between the observed spectral energy distribution of the ionizing continuum and the narrow emission line strengths. Objects with the flattest soft X-ray continua tend to possess high-column density UV absorption, and it is plausible that the differences in narrow emission line ratios among these galaxies are an indication of the effects of absorbing material internal to the narrow-line region, rather 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-line ratios and soft X-ray spectral characteristics observed in several Seyfert 1.5 galaxies. Our results also suggest that low-ionization UV absorption may be more common than currently believed.