Revisiting the giant radio galaxy ESO 422-G028 - I. Discovery of a neutral inflow and recent star formation in a restarted giant

Giant radio galaxies provide important clues into the life cycles and triggering mechanisms of radio jets. With large-scale jets spanning 1.8 Mpc, ESO 422-G028 (z = 0.038) is a giant radio galaxy that also exhibits signs of restarted jet activity in the form of pc-scale jets. We present a study of the spatially resolved stellar and gas properties of ESO 422-G028 using optical integral field spectroscopy from the Wide-Field Spectrograph (WiFeS). In addition to the majority 13\, \rm Gyr old stellar population, ESO 422-G028 exhibits a much younger ({\lesssim} 10\, \rm Myr old) component with an estimated mass of 10{7.6}\, \rm M\odot that is predominantly located in the north-west region of the galaxy. Unusually, the ionized gas kinematics reveal two distinct discs traced by narrow (\sigma {\rm H\alpha } \lt 100 \, \rm km\, s{-1}) and broad (\sigma {\rm H\alpha } \gt 150 \, \rm km\, s{-1}) Hα emission, respectively. Both ionized gas discs are misaligned with the axis of stellar rotation, suggesting an external origin. This is consistent with the prominent interstellar Na D absorption, which traces a 1 \!-\! 3 \, \rm M\odot \, yr{-1} inflow of neutral gas from the north. We posit that an inflow of gas - either from an accretion event or a gas-rich merger - has triggered both the starburst and the restarted jet activity, and that ESO 422-G028 is potentially on the brink of an epoch of powerful active galactic nucleus (AGN) activity.