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

TL;DR

This study reveals a restarted jet, recent star formation, and a neutral gas inflow in the giant radio galaxy ESO 422-G028, suggesting external gas accretion triggered both starburst and jet activity, indicating a potential new active phase.

Contribution

First detailed spatially-resolved analysis of stellar and gas properties in ESO 422-G028, linking gas inflow to recent star formation and jet restarting in a giant radio galaxy.

Findings

Detection of a young stellar component less than 10 Myr old.

Identification of two misaligned ionised gas disks with external origins.

Evidence of neutral gas inflow from Na D absorption suggesting external gas accretion.

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 $\sim 13\,\rm Gyr$ 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$ which is predominantly located in the North-West region of the galaxy. Unusually, the ionised gas kinematics reveal two distinct disks traced by narrow ($\sigma_{\rm H\alpha} < 100 \,\rm km\,s^{-1}$) and broad ($\sigma_{\rm H\alpha} > 150 \,\rm km\,s^{-1}$) 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 $1 - 3 \,\rm M_\odot \, yr^{-1}$ 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 AGN activity.