Radio Outflows and the Origin of the Narrow-Line Region in Seyfert Galaxies

Abstract

We present the results of Hubble Space Telescope medium-band imaging of four Seyfert 2 galaxies, Mrk 573, Mrk 348, Mrk 3, and Mrk 78, which show extended radio emission, and ground based Johnson A images of Mrk 3 and Mrk 78 obtained with the William Herschel Telescope. Together these two data sets allow us to investigate the relationship between the narrow-line region (NLR) emission structure, the radio structure, and the large-scale morphology of the host galaxy. In all four objects there is a close association between the NLR emission-line morphology and that of the radio emission. Furthermore, it appears that the NLR takes a different form depending on the structure of the radio emission. The emission-line regions associated with the radio lobes are shell-like 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]/Hα show only a slow increase with radius between the bow shocks ([O III]/[O II] ~3 and [O III]/Hα ~ 1.5, r < 2") and the outer filaments [O III]/[O II] ~ 4 and [O III]/Hα ~ 2.2" <r <4"). A similar, but more substantial, trend for increasing ionization with radius is seen in Mrk 78 ([O III]/[O II] ~ 1.4 in the core and [O III]/[O II]~ 2.9 in the western lobe). In the case of Mrk 573, it appears that a local source of ionization is required to explain the observed ionization structure. We interpret these results 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 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 material by the ejected and expanding lobes, while the linear structure associated with the jets arises due to the lateral expansion of hot gas around the jet axis. There is no evidence for an unresolved bright core which could be identified as the active nucleus in any of these galaxies. From this, we conclude that the nucleus must be hidden along our line of sight. In all four objects, an absorption lane crossing the nuclear region has been detected. In Mrk 348, Mrk 573, and Mrk 3, the typical scale height of the dust lane is less than 50 pc, while in Mrk 78 it is ~180 pc. We suggest that these dust lanes are associated 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 perpendicular to the outer bar. We discuss the relationship between the bars and the orientation of the NLR.