Abstracts Astronomy & Space Science

Formation of the First Galaxies

by Xi https://orcid.org/0000-0002-8276-4164 Meng

Galaxies at high redshift have different structure and star formation properties from low-redshift galaxies. While most nearby L* galaxies present disk-like morphologies, deep observations with the Hubble Space Telescope (HST) reveal irregular and clumpy shapes of high-redshift galaxies at rest-frame UV and optical wavelengths. It is also observed that high-redshift galaxies are more turbulent and have thicker disks than their low-redshift counterparts. Investigating the evolution of galaxy structure across cosmic time can reveal the interplay between star formation and the galaxy as a whole. In this thesis I explored the formation and evolution of high-redshift galaxies using cosmological simulations. I have investigated the structure of simulated high-redshift Milky Way-sized galaxies. Specifically, their irregular and prolate shapes, turbulent orbits, thick disks, and clustered and bursty star formation make their properties distinct from those of the local galaxies. Despite all these differences, I found that we could still apply Toomre stability analysis to these high-redshift galaxies. Simulations can prove nothing if they are far from the reality. Therefore, I made close comparisons of the simulated galaxies to observations. The simulations correctly reproduced the non- spatial coincidence of young stars and molecular gas, as well as the huge star-forming clumps observed in high-redshift galaxies. I compared the observed star-forming clumps with the intrinsic 3D clumps of young stars and found that the stellar masses and sizes of the observed clumps are significantly overestimated due to a combination of spatial resolution and projection. I studied the growth of the stellar disk thickness and found that in our simulations stars always form in relatively thin disks that quickly thicken in a few hundred Myr. These results validated the star formation recipe in the simulations and helped interpret high-redshift observations that are limited by resolution and sensitivity. Another way to study galaxy evolution is through massive star clusters, which formed in the high-redshift universe and could become globular clusters that are still observable today. I traced the tidal field of the massive star clusters and found that they experience the strongest tidal force in the first few hundred Myr after formation, and then plateaus at a lower value. I also found that the survival rate of clusters from different progenitors is different due to the tidal field they experience at late times. These results could be used in globular cluster formation models and help reconstruct the assembly history of our galaxy using globular clusters.