MRC B1221-423: a compact steep-spectrum radio source in a merging galaxy

We present BVRIK images and spectroscopic observations of the z = 0.17 host galaxy of the compact steep-spectrum (CSS) radio source MRCB1221-423. This is a young (similar to 10(5) yr) radio source with double lobes lying well within the visible galaxy. The host galaxy is undergoing tidal interaction with a nearby companion, with shells, tidal tails and knotty star-forming regions all visible. We analyse the images of the galaxy and its companion pixel-by-pixel, first using colour-magnitude diagrams and then fitting stellar population models to the spectral energy distributions (SEDs) of each pixel. We also present medium-resolution spectroscopy of the system. The pixels separate cleanly in colour-magnitude diagrams, with pixels of different colours occupying distinct regions of the host galaxy and its companion. Fitting stellar population models to these colours, we have estimated the age of each population. We find three distinct groups of ages: an old population (tau similar to 15 Gyr) in the outskirts of the host galaxy; an intermediate-age population (tau similar to 300 Myr) around the nucleus and tidal tail, and a young population (tau less than or similar to 10 Myr) in the nucleus and blue knots. The spectrum of the nucleus shows numerous strong emission lines, including [O I]lambda6300, [O II]lambda3727, [S II]lambdalambda6716, 6731, Halpha and [N II]lambdalambda6548, 6583, characteristic of a low-ionization nuclear emission-line region (LINER) spectrum. The companion galaxy shows much narrower emission lines with very different line ratios, characteristic of a starburst galaxy. We have evidence for three distinct episodes of star formation in B1221-423. The correlation of age with position suggests the two most recent episodes were triggered by tidal interactions with the companion galaxy. The evidence points to the active galactic nucleus (AGN) in the centre of B1221-423 having been caught in the act of ignition. However, none of the components we have identified is as young as the radio source, implying that the delay between the interaction and the triggering of the AGN is at least 3 x 10(8) yr.