  We present BVRIK images and spectroscopic observations of the z=0.17 host
galaxy of the compact steep-spectrum radio source MRC B1221-423. This is a
young (~1e5 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 of each pixel. The pixels separate
cleanly in colour-magnitude diagrams, with pixels of different colours
occupying distinct regions of the host galaxy and its companion. We find three
distinct groups of ages: an old population in the outskirts of the host galaxy;
an intermediate-age population around the nucleus and tidal tail, and a young
population in the nucleus and blue "knots".
  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 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 300 My.
