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Doctor of Philosophy (PhD)
Year Degree Awarded
Month Degree Awarded
External Galaxies | Stars, Interstellar Medium and the Galaxy
One of the major questions in observational cosmology is how galaxies formed and how they evolved. In particular, understanding the assembly history of galaxies at the peak epoch of the star formation activity, z=1-3, is a key to understanding the whole picture of the Universe, but remains uncertain. Galaxies with various physical properties and morphologies have different formation and evolution histories. As such, we seek insight into galaxy formation and evolution at z=1-3 using galaxies selected from Cosmic Assembly Near-infrared Deep Extragalactic Legacy Survey (CANDELS) in this dissertation. First, we investigate the relationship between spectral types and morphologies using various parametric diagnostics and visual inspections. Our sample clearly separates into massive, red, and passive galaxies versus less massive, blue, and star forming ones, and this dichotomy correlates very well with the galaxies' morphological properties. From this study, we suggest that the backbone of the Hubble sequence was already in place at z~2. Second, we explore how the choice of star formation histories affects estimating galaxy properties by adopting flexible star formation history models to the fitting of galaxy's spectrum. The estimation of galaxy properties is improved using CANDELS observations providing unprecedented coverage and depths, and using an advanced fitting technique. We find that galaxy properties, particularly age and star formation rate, are sensitive to the choice of star formation histories. We also find that using different best-fit star formation histories leads to significantly different results on the main sequence of star formation. Our results demonstrate that using the best-fit star formation history for each galaxy is more appropriate way than using one analytic model for all galaxy types. Third, with accurately measured stellar mass and star formation rate, we study characteristics of galaxies on, above, and below the main sequence. We find that distinct morphological differences are shown among different galaxy populations using various diagnostics. On average, as star formation activities decrease, galaxies become denser having smaller sizes and steeper light profiles at all explored redshifts. We also show that the compact morphology is not necessary to precede a passivity of star formation. Our results do not support that gas-rich merging is the key driver to assemble very compact, massive early-type galaxies observed at z~2. Instead, we suggest that compact galaxies simply assemble at very early times and evolve through in situ star formation to form compact massive, quiescent galaxies without significant merging events.
Lee, Bomee, "Intrinsic Characteristics of Galaxies in the Distant Universe: The Correlation Between Galaxy Morphology and Star Formation Activity" (2017). Doctoral Dissertations. 887.