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Galactic Chemical Evolution: Estimating the Fuel Supply Rate on the Galactic Disk from High-velocity Cloud (HVC) Infall

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Title
Galactic Chemical Evolution: Estimating the Fuel Supply Rate on the Galactic Disk from High-velocity Cloud (HVC) Infall
Author
Sung, Kwang Hyun
Advisor
Kwak, Kyujin
Keywords
Galactic Chemical Evolution; GCE; High-velocity Clouds; HVC
Issue Date
2020-02
Publisher
Graduate School of UNIST
Abstract
Driven by the stellar nucleosynthesis over numerous generations of stars, Galactic Chemical Evolution (GCE) is the proportional buildup of helium and heavier elements within a galaxy. The most evident way to reveal the history of the galactic chemical composition and distribution is to search for the stars with a very long lifetime which would serve as fossils carrying information of the chemical abundances at the time the stars were born. However, a job of collecting a good amount of stars enough to explain the complete evolution history of the chemical abundance in a galaxy is nearly impossible to accomplish. Therefore, instead of looking for fossils, previous studies focused on developing an alternative method which is to build an analytical model from the basic knowledge of physics and our galaxy. One of the first GCE models to be developed was the Simple Model of galactic chemical evolution. The history of chemical enrichment in closed systems such as the galactic bulge was fairly well reconstructed through the model. However, when applied to the nearby solar system, the number of low-metal stars were overpredicted compared to the observed number of stars. Soon after, it was proved that allowing low-metal gas inflow could be one of the solutions to such discrepancy between the model prediction and real observation and the infall rate of ~ 0.45 solar mass per year was suggested from GCE models. Later discovered through the 21 cm radio emission line, High Velocity Clouds (HVCs) with the deviation velocities over 90 km/s are considered a good supply of metal-poor gas as the estimated mass infall rate from the HVCs can be up to ~ 0.4 solar mass per year. The work introduced in this thesis is driven from a straightforward sense that the HVC infall rates could be overestimated mainly from two reasons. (1) The hydrodynamic interaction between the inflowing cloud and the galactic disk is neglected in the estimation of the infall rate. (2) The infalling complexes will not always fully and progressively accumulate on the disk. Therefore, the "fuel supply rate" is newly defined as the true amount of HVC material that is donated to the galactic disk from the infalling HVCs when the hydrodynamic interaction between the cloud and the disk is considered. A total of 4 different infall cases are constructed with 11 HVC complexes of selection to show that the infall rate is overestimated compared to the true material supply rate. From the simulation results, it is shown that the fuel supply efficiency from HVCs infall can be as low as ~ 0.042 compared to the traditional maximum accretion scenario. The fuel supply efficiency is low for the reason that the HVC complexes selected for the simulations do not permeate further through the galactic disk as the densities of the selected HVC complexes are lower than the density of the gaseous disk. The fuel efficiency is increased with the density of the cloud as the hydrodynamical interaction between the disk and the complex is considered. Also identified is that the gas density of the cloud has an impact on the fuel supply efficiency and that the effect of the density is greater than that of the velocity of the approaching complex. The simulations in this work do not include physical processes such as gravity, gravitational fields, cooling effects, and magnetic fields. On top of that, a uniform density profile is adopted for the interstellar medium (ISM), HVCs, and the galactic disk. All combined, the simulation in this study can be significantly improved for a better estimation of the true fuel supply rate. Nonetheless, the inefficiency of fuel supply through HVC material infusion that is suggested from the simulation results is valid. One of the final goals of this study is to estimate the number of stars formed from HVC infall. Required for such work is a better estimation of the HVC fuel supply rate which can be provided by implementing a non-uniform density profile for the HVC complexes and the gaseous disk, adding gravity, magnetic field, and cooling, and further by investigating on the fraction of H_2 which is converted from the amount of HI supplied from the inflow of HVCs.
Description
Department of Physics
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