Multiphase gas in the baryon cycle: from jellyfish galaxies to turbulent galactic outflows

Abstract

The formation and evolution of galaxies are shaped not only by gravity but also by the complex behaviour of gas—the dominant baryonic component within and around galaxies. This gas spans a wide range of temperatures, densities, and ionization states, forming a highly multiphase medium whose evolution is governed by hydrodynamic instabilities, radiative cooling, turbulent mixing, and momentum exchange. Unlike terrestrial fluids, these processes unfold over vast scales and within the gravitational potentials of dark matter halos and the cosmic web. Recent multiwavelength observations – from radio, optical, UV to X-ray – have revealed that galaxies are embedded in a diffuse, multiphase halo – the circumgalactic medium (CGM). The CGM acts both as a reservoir of gas and a sink for feedback-driven outflow. Supernovae or AGN-driven feedback enrich the CGM and intergalactic medium with metal-rich cold (10⁴ K) clouds embedded in hot galactic winds (∼10⁶ K). However, the small-scale processes that regulate mixing, cooling, and cloud survival remain unresolved in cosmological simulations. In this thesis, we explore the evolution of multiphase gas in galaxy halos across two diverse scales: (i) kiloparsec-scale environmental interaction of satellites in clusters, and (ii) parsec-scale cloud–wind interactions in galactic outflows. In the first part of the thesis, we examine how the cluster environment affects the CGM of satellite galaxies as they move in the intracluster medium (ICM). Using three-dimensional hydrodynamic simulations in a wind-tunnel setup, we model a massive satellite galaxy – akin to JO206 – subjected to a steady ICM wind across a wide parameter space without radiative cooling. We track the evolution of both the interstellar medium (ISM) and the surrounding CGM. Our results show that while the dense ISM remains bound due to the galaxy’s gravitational potential, the more diffuse CGM is susceptible to stripping. The CGM behaves like a low-density bubble accelerated by the wind, with its removal governed primarily by hydrodynamic drag forces. In the second part, we explore the role of radiative cooling in the formation and survival of cold gas in the stripped tails of satellite galaxies. We address whether the cold phase arises solely from the direct stripping of ISM or can also form through the condensation of gas from the CGM and ICM. Our simulations reveal that while early tail formation in such “jellyfish” galaxies is dominated by stripped ISM, radiative cooling enables the condensation of CGM and ICM gas at later times - highlighting a multiphase origin for the cold gas observed in these galaxies. In the third part, we perform a suite of high-resolution cloud-crushing simulations to study cold cloud evolution in turbulent galactic winds. Unlike prior studies limited to laminar flows, our simulations incorporate continuous external turbulent forcing, capturing more realistic wind conditions. We explore a broad parameter space defined by the mixing timescale from shear instabilities, the cooling time of gas formed at intermediate temperatures, and the turbulent Mach number. Our work suggests that (subsonic) turbulence can either enhance or suppress cloud survival, depending on the cooling regime. When cooling is efficient, turbulence increases the cloud’s surface area, boosting mixing and leading to growth in cold gas mass.

Overall, this thesis emphasizes the crucial role of small-scale hydrodynamic processes in shaping the evolution of gas across different phases and scales. Our results provide important insights for modelling the baryon cycle in galaxies and for interpreting the complex, multiphase gas structures observed in galaxy halos and outflows.