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 ( \sim 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 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 \sim 15 Gyr ) in the outskirts of the host galaxy ; an intermediate-age population ( \tau \sim 300 Myr ) around the nucleus and tidal tail , and a young population ( \tau \mathrel { \hbox { \raise 2.15 pt \hbox { $ < $ } \hbox to 0.0 pt { \lower 2.15 pt \hbox { $% \sim$ } } } } 10 Myr ) in the nucleus and blue “ knots ” .

The spectrum of the nucleus shows numerous strong emission lines , including [ O i ] \lambda 6300 , [ O ii ] \lambda 3727 , [ S ii ] \lambda \lambda 6716 , 6731 , H \alpha , and [ N ii ] \lambda \lambda 6548 , 6583 , characteristic of a 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 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 \times 10 ^ { 8 } years .