The role of stratifcation, turbulence and cooling in the intracluster medium

Abstract

The intracluster medium (ICM) refers to the gas pervading the region between galaxies in a galaxy cluster. It comprises multiple temperature phases with a dominant hot phase at 10-100 million K and cooler atomic and molecular phases at around 10,000 K and around 10 K, respectively. It is intimately linked to the galaxies in the cluster through cycles of gas cooling and inflow into the central galaxy, and outflows into the ICM driven by active galactic nuclei (AGNs) and supernovae fed by this cooling gas. These feedback processes as well as galaxy-ICM interactions and cluster mergers drive turbulence in the ICM, which plays a critical role in the cosmic baryon cycle. For example, it can seed thermal instability by generating density fluctuations, enhance the mixing between the gas at different temperatures and heat the ICM through viscous dissipation. Due to the lack of sufficient spectral resolution of current X-ray telescopes, we are unable to directly measure the turbulent gas velocities in the ICM of most clusters. However, the turbulent motions are associated with fluctuations in the gas density and pressure, which manifest as perturbations in X-ray surface brightness (SB) and as thermal Sunyaev-Zeldovich effect (tSZ) fluctuations, respectively. Using integral field unit (IFU) spectroscopy, the velocities of the the cold atomic phase gas in the central regions of the ICM have now been obtained. In this thesis, I study the relation between these different observable properties of the ICM and the turbulent velocities of the hot ICM. I study turbulence in the ICM using idealised hydrodynamical simulations. The ICM is density-stratified, with the gas density being highest at the centre of the cluster and decreasing radially outwards. In my first two studies, I conduct simulations of turbulence in a stratified medium. I propose scaling relations between the density and pressure fluctuations with the rms Mach number (ratio of the turbulent speed to the speed of sound) and the Froude number (which denotes the degree of stratification) which hold for typical ICM conditions. These are useful to estimate the turbulent velocities of the ICM using fluctuations in the X-ray surface brightness and the tSZ effect. In my next set of studies, I simulate the ICM with radiative cooling and study the effect of varying levels of turbulence. I find that upon increasing the strength of turbulence driving, mixing between the hot and cold gas becomes stronger, leading to more gas at intermediate temperatures (between 10, 000 K and 10 million K). I conclude that a weak turbulence scenario best explains the subsonic gas motions observed in the ICM. I study the relation between velocity fluctuations of the hot and cold phases, which will help interpret recent observations of optical filaments in nearby clusters. The ICM exhibits strong density variations such as shocks, cold fronts and radio relics, which could be caused by compressive motions. I investigate the effect of the nature of turbulence driving on the ICM gas properties. I find that compressive driving generates stronger large-scale density and pressure fluctuations compared to solenoidal driving. The cold phase gas forms large-scale filamentary structures for compressive driving and misty, small-scale clouds for solenoidal driving. Many of my results can be directly tested by XRISM, the X-ray telescope which is expected to obtain direct measurements of turbulent gas velocities of the hot ICM. In addition, the eROSITA X-ray survey would significantly increase our sample of known clusters. Upcoming cosmic microwave background (CMB) surveys by the Atacama Cosmology telescope and the South Pole Telescope will obtain high resolution tSZ effect data on many clusters. The results of this PhD will be useful towards combining multi-wavelength observations and constrain cluster properties, set to become one of the frontiers of modern astronomy. Show more