The Narrow‐Line Region of Narrow‐Line Seyfert 1 Galaxies

This work studies the optical emission line properties and physicalconditions of the narrow line region (NLR) of seven narrow-line Seyfert 1galaxies (NLS1). Our results show that the flux carried out by the narrowcomponent of H-beta is, on average, 50% of the total line flux. As a result,the [OIII] 5007/H-beta ratio emitted in the NLR varies from 1 to 5, instead ofthe universally adopted value of 10. This has strong implications for therequired spectral energy distribution that ionizes the NLR gas. Photoionizationmodels that consider a NLR composed of a combination of matter-bounded andionization-bounded clouds are successful at explaining the low [OIII]5007/H-beta ratio and the weakness of low-ionization lines of NLS1s. Variationof the relative proportion of these two type of clouds nicely reproduce thedispersion of narrow line ratios found among the NLS1 sample. Assuming similarphysical model parameters of both NLS1s and the normal Seyfert 1 galaxy NGC5548, we show that the observed differences of emission line ratios betweenthese two groups can be explained in terms of the shape of the input ionizingcontinuum. Narrow emission line ratios of NLS1s are better reproduced by asteep power-law continuum in the EUV -- soft X-ray region, with spectral indexalpha ~ -2. Flatter spectral indices (alpha ~ -1.5) match the observed lineratios of NGC 5548 but are unable to provide a good match to the NLS1 ratios.This result is consistent with ROSAT observations of NLS1s, which show thatthese objects are characterized by steeper power-law indices than those of Sy1galaxies with strong broad optical lines.Comment: Accepted to The Astrophysical Journal. AASTeX, 16 pages, 7 figure