Galactic interaction as the trigger for the young radio galaxy MRC B1221−423★

Abstract

Mergers between a massive galaxy and a small gas-rich companion (minor mergers) have been proposed as a viable mechanism for triggering radio emission in an active galaxy. Until now the problem has been catching this sequence of events as they occur. With MRC B1221$-$423 we have an active radio galaxy that has only recently been triggered, and a companion galaxy that provides the "smoking gun". Using spectroscopic data taken with the VIMOS Integral Field Unit detector on the European Southern Observatory's Very Large Telescope, we have examined the distribution, ionization state and kinematics of ionized gas in this interacting system. We have also modelled the stellar continuum with synthesised spectra of stellar populations of different ages. From our study of the ionized gas, we have derived preliminary models for the geometry of the interaction, analysed the kinematic behaviour of the ionized gas, and examined the ionization mechanisms at work throughout the system. Our modelling of the stellar continuum allowed us to identify and date distinct stellar populations within the galaxy pair. We find evidence of multiple episodes of widespread starburst activity, and by dating these populations, we provide tentative insight into the history of the interaction.