Jets blowing bubbles in the young radio galaxy 4C 31.04

We report the discovery of shocked molecular and ionized gas resulting from jet-driven feedback in the compact radio galaxy 4C 31.04 using near-IR imaging spectroscopy. 4C 31.04 is a $\sim 100$ pc double-lobed Compact Steep Spectrum source believed to be a very young AGN. It is hosted by a giant elliptical with a $\sim {10}^9{\mathrm{M}}_{\odot }$ multi-phase gaseous circumnuclear disc. We used high spatial resolution, adaptive optics-assisted $H$- and $K$-band integral field Gemini/NIFS observations to probe (1) the warm ($\sim {10}^3\mathrm{K}$) molecular gas phase, traced by ro-vibrational transitions of ${\mathrm{H}}_2$, and (2), the warm ionized medium, traced by the [Fe II]${}_{1.644\mu \mathrm{m}}$ line. The [Fe II] emission traces shocked gas ejected from the disc plane by a jet-blown bubble $300-400\mathrm{p}\mathrm{c}$ in diameter, whilst the ${\mathrm{H}}_2$ emission traces shock-excited molecular gas in the interior $\sim 1\mathrm{k}\mathrm{p}\mathrm{c}$ of the circumnuclear disc. Hydrodynamical modelling shows that the apparent discrepancy between the extent of the shocked gas and the radio emission can occur when the brightest regions of the synchrotron-emitting plasma are temporarily halted by dense clumps, whilst less bright plasma can percolate through the porous ISM and form an energy-driven bubble that expands freely out of the disc plane. This bubble is filled with low surface-brightness plasma not visible in existing VLBI observations of 4C 31.04 due to insufficient sensitivity. Additional radial flows of jet plasma may percolate to $\sim \rm kpc$ radii in the circumnuclear disc, driving shocks and accelerating clouds of gas, giving rise to the ${\mathrm{H}}_2$ emission.