Simulating the Cosmic Gas: From Globular Clusters to the Most Massive Haloes

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Simulating the Cosmic Gas: From Globular Clusters to the Most Massive Haloes

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Title: Simulating the Cosmic Gas: From Globular Clusters to the Most Massive Haloes
Author: Popa, Cristina
Citation: Popa, Cristina. 2016. Simulating the Cosmic Gas: From Globular Clusters to the Most Massive Haloes. Doctoral dissertation, Harvard University, Graduate School of Arts & Sciences.
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Abstract: In spite of the rapid progress of numerical simulations as tools for studying large-scale structure formation, there is still a considerable degree of uncertainty regarding the details of the physical processes governing the dynamics of the baryonic matter in the Universe. In this thesis, we explore the impact that specific choices in the modeling of baryonic physics in computer simulations have on the gas content of haloes spanning more than 10 order of magnitude in mass, from the size of globular clusters with masses in the $10^4-10^7M_{\astrosun}$ range, to the most massive galaxy clusters at $10^{15}M_{\astrosun}$.

We start by examining the assumptions necessary in order to consistently set up the initial conditions of cosmological simulations following the evolution of both baryons and dark matter. Using adiabatic high-resolution numerical simulations, we quantify the effect that the non-negligible relative motion of baryons with respect to dark matter at the time of recombination has on structure formation and evolution. We find that a non-zero relative velocity has a sizable impact on the number density of haloes with masses $\lesssim$ few$\times 10^7 M_{\astrosun}$ up to $z=10$, the final redshift of our simulations. Furthermore, the gas stream velocity induces a suppression of the gas fraction in haloes, which at z=10 is $\sim 10 \%$ for objects with $M\sim10^7M_{\astrosun}$, as well as a flattening of the gas density profiles in the inner regions of haloes. We further identify and study the formation of moderately long lived gas dominated structures at intermediate redshifts $10 < z < 20$, that recent analytical work has proposed as potential progenitors of globular clusters.

For the remainder of the thesis, we change our focus and investigate the impact on the baryonic content of haloes of physical processes which become relevant at later stages of structure formation. Using the Illustris simulations suite, a high-resolution numerical simulation of a cosmological volume of $(106.5\,\rm{Mpc})^3$, we analyze the distribution of the ICM gas in galaxy clusters with virial masses in the $10^{13}M_{\astrosun} - 2\times10^{14}M_{\astrosun}$ range. We find a substantial spread in both structural and observable halo characteristics, particularly for the less massive objects studied, and pinpoint the source of the scatter to be the radio mode of the AGN feedback implemented. Our halo sample also indicates that the impact of AGN feedback decreases for the more massive objects, which motivates us to explore the evolution of clusters with masses above those realized in Illustris.

We further pursue the investigation of the impact of the specific implementation of AGN feedback on the properties of galaxy clusters by running and analyzing the iClusters suite, a series of high resolution zoom-in simulations of 6 galaxy clusters with masses between $2\times10^{13}M_{\astrosun}$ and $2\times10^{15}M_{\astrosun}$. Their evolution is modeled by using five distinct implementations of relevant physical processes, three of which include different AGN feedback models, recently used in the literature. We confirm a decreasing trend in the impact of AGN feedback on the state and observational signatures of intracluster gas with increasing halo mass. Furthermore, we find an excellent agreement with observations for all of the models of baryonic physics implemented in the iClusters simulations, which we believe is due to the fact that observations currently have access to objects with masses above $10^{14}M_{\astrosun}$. Finally, our simulations suggest that observations of lower mass objects are needed in order to place constraints on the specific details of the implementation of AGN feedback.
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