Active Galactic Nuclei Feedback in Multiphase Gaseous Halos

Abstract

In this thesis, I study the evolution of multiphase gaseous halos of galaxy clusters, groups, and giant elliptical galaxies using numerical simulations. I investigate how the key physical processes and model ingredients, including AGN feedback, cosmic rays, magnetic fields, and turbulence, play a role in the evolution of these systems. I perform hydrodynamical simulations to study the observed “multiphase” and “single- phase” dichotomy in giant elliptical galaxies. The multiphase galaxies possess hot gaseous halos conducive to the development of thermal instability. These galaxies are characterized by the presence of extended cold gas filaments. The gaseous halos in the single-phase galaxies are less likely to develop thermal instability and cold gas is either absent or only located in the nuclear region in such galaxies. My simulations reproduce such “multiphase” and “single-phase” dichotomy in agreement with the observations. Importantly, I find that self-regulated AGN feedback maintains the multi- or single- phase nature of the halos but does not turn multiphase galaxies into single-phase ones or vice versa. The long-term evolution of the simulated multiphase and single-phase galaxies reveals the formation of long-lived massive cold disks. Such disks are also seen in many other numerical simulations in the literature but are in tension with the observations. Thus, I explore a possible solution to this “disk problem” and find that magnetic fields can offer one such solution. The magnetic fields are overall weak in the hot halo but are locally amplified in the cold gas. The amplified magnetic fields effectively reduce the angular momentum of the cold gas, thus preventing the formation of the problematic disks. Additionally, I find that when the plasma composition in the AGN jets is dominated by cosmic rays, and when cosmic ray transport is included, the hot gaseous halos can maintain global thermal equilibrium and the massive cold central disks do not form. In this case, the power of the AGN feedback is reduced as its energy is utilized more efficiently to heat the ambient gas compared to the case of purely kinetic AGN feedback. Motivated by my finding that the magnetic fields have important dynamical impact on the cold gas, and by recent observational work on probing the ICM turbulence using the velocity structure function of the cold ICM filaments, we study the properties of turbulence in the multiphase ICM by calculating the velocity structure functions of different phases in the simulated ICM affected by the AGN feedback. We find that while the simulated turbulence is not consistent with the predictions from the classic Kolmogorov theory of turbulence, it is broadly consistent with the observations. We suggest a novel hypothesis supported by our simulations, that the turbulence of hot gas at the ICM center is driven both by the AGN outflows and the cold filaments formed via local thermal instability. Finally, I study the energetic impact of turbulence on counterbalancing the radiative cooling in the ICM. I present an analytic model that quantitatively describes the heating rate due to turbulent dissipation and turbulent mixing. In this work, I elucidate the important role of gravitational stratification in regulating these heat rates.