The Narrow Line Region of Ark 564

Abstract

The continuum and emission-line spectrum of the narrow-line Seyfert 1 galaxy Ark 564 is used to investigate, for the first time, the physical conditions and structure of its narrow line region (NLR). For this purpose, composite models, accounting for the coupled effect of photoionization and shocks, are employed. The emission-line spectrum of Ark 564, which ranges from the ultraviolet to the near-infrared, shows a rich forbidden line spectrum. Strong emphasis is given to the study of the coronal line region. The diversity of physical conditions deduced from the observations requires multi-cloud models to reproduce the observed lines and continuum. We find that a combination of high velocity (Vs = 1500 km/s) shock-dominated clouds as well as low velocity (Vs = 150 km/s) radiation-dominated clouds explains the coronal lines, while the optical low-ionization lines are mainly explained by shock-dominated clouds. The results for Ark 564 are compared with those obtained for other Seyfert galaxies previously analyzed such as NGC 5252, Circinus, NGC 4051 and NGC 4151. The model results for the ultraviolet and optical permitted lines suggest that the broad line region may contribute up to 80%, depending on the emission-line, being of about 30% for Hbeta. The consistency of the multi-cloud model is checked by comparing the predicted and observed continuum, from radio to X-ray, and indicate that the dust-to-gas ratio in the clouds varies from 10^{-15} to 10^{-12}.