Shocks, clouds, and atomic outflows in active galactic nuclei hosting relativistic jets

Abstract

Context. A number of observations have revealed atomic and/or molecular lines in active galaxies hosting jets and outflows. Line widths indicate outward motions of hundreds to a few thousands of kilometers per second. They appear to be associated with the presence of radio emission in Gigahert-peaked spectrum (GPS) and/or compact steep spectrum (CSS) sources, with linear sizes of ≤10 kpc. Numerical simulations have shown that the bow shocks triggered by relativistic jets in their host galaxies drive ionization and turbulence in the interstellar medium (ISM). However, the presence of atomic lines requires rapid recombination of ionized gas, which appears challenging to explain from the physical conditions revealed thus far based on numerical simulations of powerful jets. Aims. The aim of this paper is to provide a global framework to explain the presence of lines in terms of jet and shock evolution and to fix the parameter space where the atomic and molecular outflows might occur. Methods. This parameter space is inspired by numerical simulations and basic analytical models of jet evolution as a background. Results. Our results show that a plausible general explanation involves momentum transfer and heating to the interstellar medium gas by jet triggered shocks within the inner kiloparsecs. The presence of post-shock atomic gas is possible in the case of shocks interacting with dense clouds that remain relatively stable after the shock passage. Conclusions. According to our results, current numerical simulations cannot reproduce the physical conditions to explain the presence of atomic and molecular outflows in young radio sources. However, I show that these outflows might occur in low-power jets at all scales and I predict a trend towards powerful jets showing lines at CSS scales, when clouds have cooled to recombination temperatures.