The disc-dominated host galaxy of FR-I radio source B2 0722+30

We present new observational results that conclude that the nearby radio galaxy B2 0722+30 is one of the very few known disc galaxies in the low–redshift Universe that host a classical double–lobed radio source. In this paper, we use H I observations, deep optical imaging, stellar population synthesis modelling and emission–line diagnostics to study the host galaxy, classify the active galactic nucleus (AGN) and investigate environmental properties under which a radio–loud AGN can occur in this system. Typical for spiral galaxies, B2 0722+30 has a regularly rotating gaseous disc throughout which star formation occurs. Dust heating by the ongoing star formation is likely responsible for the high infrared luminosity of the system. The optical emission–line properties of the central region identify a Low Ionization Nuclear Emission–line Region (LINER)–type nucleus with a relatively low [O III] luminosity, in particular when compared with the total power of the Fanaroff & Riley type–I radio source that is present in this system. This classifies B2 0722+30 as a classical radio galaxy rather than a typical Seyfert galaxy. The environment of B2 0722+30 is extremely H I–rich, with several nearby interacting galaxies. We argue that a gas–rich interaction involving B2 0722+30 is a likely cause for the triggering of the radio AGN and/or the fact that the radio source managed to escape the optical boundaries of the host galaxy.