Author: Moupiya Maji

Publisher:

Published: 2018

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Over the past decade, advances in computational architecture have made it possiblefor the first time to investigate some of the fundamental questions around the birthand the growth of the building blocks of the universe; star clusters and galaxies. Inthese stellar and star-forming systems, baryonic physics play an important role indetermining their formation and evolution. Therefore, in my research, I have exploredstar-forming systems using high resolution baryonic cosmological simulations andexplored the origin of star clusters, anisotropic spatial distribution of satellite galaxiesand the effect of reionization on the evolution of dwarf galaxies.Observations of globular clusters show that they have universal lognormal massfunctions with a characteristic peak at 2 10^5MSun , although the origin of this peakeddistribution is unclear. Here I have investigated the formation and evolution of starclusters (SCs) in interacting galaxies using high-resolution hydrodynamical simulationsperformed with two different codes. I have found that massive star clusters in therange of 10^5.5 10^7.5 MSun form preferentially in extremely high-pressure gas cloudsin highly-shocked regions produced by galaxy interactions. These findings provide thefirst simulation confirmation of the analytical theory of high pressure induced clusterformation. Furthermore, these massive star clusters have quasi-lognormal initial massfunctions with a peak around 106 M . The number of clusters declines with timedue to destructive processes, but the shape and the peak of the mass functions do notchange significantly during the course of galaxy collisions. These results suggest thatgas-rich galaxy mergers provide a favorable environment for the formation of globularclusters and that the lognormal mass functions and the unique peak may originatefrom the extreme high-pressure conditions of the birth clouds and may survive thedynamical evolution.Observations of classical Milky Way satellites suggest that they are aligned in aplane inclined to the Galactic stellar disk, a phenomenon which later became knownas the disk of satellites(DoS). However, N-body simulations of galaxies predict anisotropic distribution of subhalos around the host galaxy and this discrepancy hasbeen strongly criticized as a failure of CDM. In this thesis, I have explored this highlydebated topic by reanalyzing the observations and exploring the satellite distributions in high-resolution baryonic simulations. In particular, I have demonstrated that asmall sample size can artificially produce a highly anisotropic spatial distributionand a strong clustering of the angular momenta of the satellites and have shownthat the current Milky way DoS is transient. Furthermore, I have analyzed twocosmological simulations using the same initial conditions of a Milky-Way-sizedgalaxy, an N-body run with dark matter only, and a hydrodynamic one with bothbaryonic and dark matter, and found that the hydrodynamic simulation producesa more anisotropic distribution of satellites than the N-body one. These resultssuggest that an anisotropic distribution of satellites in galaxies can originate frombaryonic processes in the hierarchical structure formation model, but the claimedhighly flattened, coherently rotating DoS of the Milky Way may be biased by the small number selection effect. Finally, I have investigated the distribution and kinematicsof satellites around a large sample of few thousand host galaxies in the Illustrissimulation and found that the DoS become more isotropic with increasing numberof satellites and no clear coherent rotation is found in most ( 90%) of the satellitesystems. Furthermore, their overall evolution indicates that the DoS may be part oflarge scale filamentary structure. These findings can help resolve the contradictoryclaims of DoS in galaxies and show that baryonic processes may be the key to solvethe so-called small scale CDM problems.Additionally, I have also explored the effects of reionization on the star formationhistories of dwarfs galaxies, using a cosmological hydrodynamic simulation of MilkyWay and its satellite galaxies. I have found that most dwarfs are extremely old systemsand star formation is quenched earlier in lower mass galaxies. During reionization,most of the lower mass dwarfs are destroyed while the remaining massive dwarfsbecome severely baryon deficient. The dwarf galaxies play a very important role inshaping the path of cosmic history, especially in terms of reionization. Observingand studying the ultra-faint dwarfs hold the key to understanding the physics of earlyuniverse in great depth.