The physics of the extended narrow-line region in active galactic nuclei

The extended narrow-line region (ENLR) is one of the most interesting and less studied structures typical of active galactic nuclei (AGN). It is made of gas, ionized by the radiation produced in the inner region of the AGN. Its spectral properties are similar to those of the narrow-line region (NLR), but the extension of ENLR is usually larger than 1 kpc (there are objects with ENLR larger than 20 kpc) while the NLR is often concentrated in a radius of some hundreds parsecs from the AGN. In this work I used optical spectra and radio and optical images to investigate the properties of some of these peculiar structures. In particular, I firstly used high resolution spectra and models combining photo-ionization and shocks to study the physical conditions of the gas in the ENLR of two nearby sources, NGC 7212 and IC 5063. The spectral resolution of the data allowed to resolve all the observed lines, which show a complex profile characterized by multiple peaks, asymmetries and bumps. Moreover, it allowed me to investigate the behavior of the gas properties as a function of its velocity. In this way I discovered that, in both AGN, shocks might give an important contribution to the ionization of the gas moving at high velocities. This property, together with the complex kinematics of the gas, might be the result of the interaction between the AGN jets and the interstellar medium (ISM) of the galaxies. Then, I started investigating the relation between extended optical emission and extended radio emission. For this reason, I looked for new extended radio structures in a VLA survey at 5 GHz of a sample of AGN belonging to the peculiar class of narrow-line Seyfert 1 galaxies (NLS1s). The first and most interesting result of this survey was the discovery of a very interesting radio emission, probably a relic, in Mrk 783, a nearby NLS1. An optical follow-up of the source revealed the presence of an ENLR. The structure is one of the largest discovered so far, with a maximum extension of 38 kpc just aligned with the most extended part of the radio emission. The host galaxy also shows signs of a recent merging with a companion, and it might be in the first stages of interaction with another nearby source.