Revisiting the Giant Radio Galaxy ESO 422-G028: Part I. Discovery of a neutral inflow and recent star formation in a restarted giant

Abstract

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 WiFeS spectrograph. In addition to the majority $\sim 13\mathrm{Gyr}$ old stellar population, ESO 422-G028 exhibits a much younger ($\lesssim 10\mathrm{Myr}$ old) component with an estimated mass of $10^{7.6}{\mathrm{M}}_{\odot }$ which is predominantly located in the North-West region of the galaxy. Unusually, the ionised gas kinematics reveal two distinct disks traced by narrow (${\sigma }_{\mathrm{H}\alpha }<100\mathrm{km}{\mathrm{s}}^{-1}$) and broad (${\sigma }_{\mathrm{H}\alpha }>150\mathrm{km}{\mathrm{s}}^{-1}$) H$\alpha$ emission respectively. Both ionised gas disks 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 $1-3{\mathrm{M}}_{\odot }\mathrm{y}{\mathrm{r}}^{-1}$ 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 AGN activity.

21 pages, 19 figures; submitted to the Monthly Notices of the Royal Astronomical Society

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