Turbulent circumnuclear disc and cold gas outflow in the newborn radio source 4C 31.04

Abstract

We present deep kpc- and pc-scale neutral atomic hydrogen (HI) absorption observations of a very young radio source (< 5000 yrs), 4C 31.04, using the WSRT and the Global VLBI array. Using $z=0.0598$, we detect a broad absorption feature centred at the systemic velocity, and narrow absorption redshifted by 220 km/s both previously observed. Additionally, we detect a new blueshifted, broad, shallow absorption wing. At pc scales, the broad absorption at the systemic velocity is detected across the entire radio source while the shallow wing is only seen against part of the eastern lobe. The velocity dispersion of the gas is overall high ($\geq$40 km/s), and is highest (>60 km/s) in the region including the outflow and the radio hot spot. While we detect a velocity gradient along the western lobe and parts of the eastern lobe, most of the gas along the rest of the eastern lobe exhibits no signs of rotation. We therefore conclude that the radio lobes of 4C 31.04 are expanding into a circumnuclear disc, partially disrupting it and making the gas highly turbulent. The distribution of gas is predominantly smooth at the spatial resolution of ~4 pc studied here. However, clumps of gas are also present, particularly along the eastern lobe. This lobe is strongly interacting with the clouds and driving an outflow ~35 pc from the radio core, with a mass-outflow rate of $0.3 \leq \dot{M} \leq 1.4$ M$_\odot$/yr. We compare our observations with a model on the survival of atomic gas clouds in radio-jet-driven outflows and find that the existence of a sub-kpc outflow implies high gas density and inefficient mixing of the cold gas with the hot medium, leading to shorter cooling times. Overall, this provides further evidence of the strong impact of young radio jets on cold ISM and supports the predictions of simulations regarding jet$-$ISM interactions and the nature of the gas into which the jets expand.