When galaxies collide: understanding the broad absorption-line radio galaxy 4C +72.26

We present a range of new observations of the "broad absorption line radio galaxy" 4C +72.26 (z = 3.5), including sensitive rest-frame ultraviolet integral field spectroscopy using the Gemini/GMOS-N instrument and Subaru/CISCO K-band imaging and spectroscopy. We show that 4C +72.26 is a system of two vigorously star-forming galaxies superimposed along the line of sight separated by ~1300 +/- 200 km/s in velocity, with each demonstrating spectroscopically resolved absorption lines. The most active star-forming galaxy also hosts the accreting supermassive black hole which powers the extended radio source. We conclude that the star formation is unlikely to have been induced by a shock caused by the passage of the radio jet, and instead propose that a collision is a more probable trigger for the star formation. Despite the massive starburst, the UV-mid-infrared spectral energy distribution suggests that the pre-existing stellar population comprises ~10^12 Msolar of stellar mass, with the current burst only contributing a further ~2%, suggesting that 4C +72.26 has already assembled most of its final stellar mass.