The Hst View of the Narrow Line Region of Seyfert Galaxies

HST observations provided us with the spatial resolution required to explore in detail the properties of the Narrow Line Region of Seyfert galaxies. In Seyferts with extended radio structures the HST images revealed a very close connection between radio and optical line emission. We interpret this result as strong evidence that the line-emitting gas is compressed by the shocks created by the passage of the radio-emitting outflow. The increase in the density due to the shocks causes the line emission to be highly enhanced in the region where this interaction occurs. Spectroscopy of the NLR confirms the presence of a strong interaction: in the region co-spatial with the radio-jets, the gas velocity field is highly perturbed and shows two velocity systems separated by as much as 1700km s. In several locations the split lines form an almost complete velocity ellipsoid, implying that we are seeing an expanding shell of gas. We conclude that the morphology and the kinematics of the NLR in these sources are dominated by the presence of radio outflows, which might also have a significant role in the ionization balance. Observations of Seyfert galaxies represent a unique tool to study energetics and evolution of ISM/jets systems.