RADIO OUTFLOWS AND THE ORIGIN OF THE NARROW-LINE REGION IN SEYFERT-GALAXIES

We present the results of Hubble Space Telescope medium-band imaging o f four Seyfert 2 galaxies, Mrk 573, Mrk 348, Mrk 3, and Mrk 78, which show extended radio emission, and ground based Johnson R images of Mrk 3 and Mrk 78 obtained with the William Herschel Telescope. Together t hese two data sets allow us to investigate the relationship between th e narrow-line region (NLR) emission structure, the radio structure, an d the large-scale morphology of the host galaxy.In all four objects th ere is a close association between the NLR emission-line morphology an d that of the radio emission. Furthermore, it appears that the NLR tak es a different form depending on the structure of the radio emission. The emission-line regions associated with the radio lobes are shell-li ke or bow shock-like, while those associated with the jets are linear. In Mrk 573, the emission-line ratios [O III]/[O II] and [O III]/H alp ha show only a slow increase with radius between the bow shocks ([O II I]/[O II] similar to 3 and [O III]/H alpha similar to 1.5, r < 2 '') a nd the outer filaments [O III]/[O II] similar to 4 and [O III]/H alpha similar to 2, 2 '' < r < 4 ''). A similar, but more substantial, tren d for increasing ionization with radius is seen in Mrk 78 ([O III]/[O II] similar to 1.4 in the core and [O III]/[O II] similar to 2.9 in th e western lobe). In the case of Mrk 573, it appears that a local sourc e of ionization is required to explain the observed ionization structu re. We interpret these results as strong evidence that the line-emitti ng gas is compressed by the shocks created by the passage of the radio -emitting outflow. The increase in the density due to the shocks cause s the line emission to be highly enhanced in the region in which this interaction occurs. The shell-like morphology of the emission-line gas associated with the radio lobes is formed by the sweeping-up of mater ial by the ejected and expanding lobes, while the linear structure ass ociated with the jets arises due to the lateral expansion of hot gas a round the jet axis. There is no evidence for an unresolved bright core which could be identified as the active nucleus in any of these galax ies. From this, we conclude that the nucleus must be hidden along our line of sight. In all four objects, an absorption lane crossing the nu clear region has been detected. In Mrk 348, Mrk 573, and Mrk 3, the ty pical scale height of the dust lane is less than 50 pc, while in Mrk 7 8 it is similar to 180 pc. We suggest that these dust lanes are associ ated with the obscuring torus which occults the Seyfert nucleus. Three of the objects (Mrk 573, Mrk 3, and Mrk 78) show large-scale stellar bars, and Mrk 573 also shows an inner bar oriented nearly perpendicula r to the outer bar. We discuss the relationship between the bars and t he orientation of the NLR.