Illuminating The Star Clusters And Dwarf Galaxies by Multi-scale Baryonic Simulations

Open Access
Maji, Moupiya
Graduate Program:
Astronomy and Astrophysics
Doctor of Philosophy
Document Type:
Date of Defense:
June 12, 2018
Committee Members:
  • Yuexing Cindy Li, Dissertation Advisor/Co-Advisor
  • Yuexing Cindy Li, Committee Chair/Co-Chair
  • Robin Bruce Ciardullo, Committee Member
  • Jane Camilla Charlton, Committee Member
  • Sarah Elizabeth Shandera, Outside Member
  • Donghui Jeong, Committee Member
  • galaxies
  • star clusters
  • computational astrophysics
  • globular clusters
  • dwarf galaxies
  • milky way
Over the past decade, advances in computational architecture have made it possible for the first time to investigate some of the fundamental questions around the birth and the growth of the building blocks of the universe; star clusters and galaxies. In these stellar and star-forming systems, baryonic physics play an important role in determining their formation and evolution. Therefore, in my research, I have explored star-forming systems using high resolution baryonic cosmological simulations and explored the origin of star clusters, anisotropic spatial distribution of satellite galaxies and the effect of reionization on the evolution of dwarf galaxies. Observations of globular clusters show that they have universal lognormal mass functions with a characteristic peak at 2 × 10^5MSun , although the origin of this peaked distribution is unclear. Here I have investigated the formation and evolution of star clusters (SCs) in interacting galaxies using high-resolution hydrodynamical simulations performed with two different codes. I have found that massive star clusters in the range of ∼ 10^5.5 − 10^7.5 MSun form preferentially in extremely high-pressure gas clouds in highly-shocked regions produced by galaxy interactions. These findings provide the first simulation confirmation of the analytical theory of high pressure induced cluster formation. Furthermore, these massive star clusters have quasi-lognormal initial mass functions with a peak around ∼ 106 M . The number of clusters declines with time due to destructive processes, but the shape and the peak of the mass functions do not change significantly during the course of galaxy collisions. These results suggest that gas-rich galaxy mergers provide a favorable environment for the formation of globular clusters and that the lognormal mass functions and the unique peak may originate from the extreme high-pressure conditions of the birth clouds and may survive the dynamical evolution. Observations of classical Milky Way satellites suggest that they are aligned in a plane inclined to the Galactic stellar disk, a phenomenon which later became known as the “disk of satellites”(DoS). However, N-body simulations of galaxies predict an isotropic distribution of subhalos around the host galaxy and this discrepancy has been strongly criticized as a failure of ΛCDM. In this thesis, I have explored this highly debated topic by reanalyzing the observations and exploring the satellite distributions in high-resolution baryonic simulations. In particular, I have demonstrated that a small sample size can artificially produce a highly anisotropic spatial distribution and a strong clustering of the angular momenta of the satellites and have shown that the current Milky way DoS is transient. Furthermore, I have analyzed two cosmological simulations using the same initial conditions of a Milky-Way-sized galaxy, an N-body run with dark matter only, and a hydrodynamic one with both baryonic and dark matter, and found that the hydrodynamic simulation produces a more anisotropic distribution of satellites than the N-body one. These results suggest that an anisotropic distribution of satellites in galaxies can originate from baryonic processes in the hierarchical structure formation model, but the claimed highly flattened, coherently rotating DoS of the Milky Way may be biased by the small number selection effect. Finally, I have investigated the distribution and kinematics of satellites around a large sample of few thousand host galaxies in the Illustris simulation and found that the DoS become more isotropic with increasing number of satellites and no clear coherent rotation is found in most (∼ 90%) of the satellite systems. Furthermore, their overall evolution indicates that the DoS may be part of large scale filamentary structure. These findings can help resolve the contradictory claims of DoS in galaxies and show that baryonic processes may be the key to solve the so-called small scale ΛCDM problems. Additionally, I have also explored the effects of reionization on the star formation histories of dwarfs galaxies, using a cosmological hydrodynamic simulation of Milky Way and its satellite galaxies. I have found that most dwarfs are extremely old systems and star formation is quenched earlier in lower mass galaxies. During reionization, most of the lower mass dwarfs are destroyed while the remaining massive dwarfs become severely baryon deficient. The dwarf galaxies play a very important role in shaping the path of cosmic history, especially in terms of reionization. Observing and studying the ultra-faint dwarfs hold the key to understanding the physics of early universe in great depth.