Galaxies in the Young Universe: Structures, Masses, and Composition of Star-Forming Galaxies at z~1.5-3

Understanding the physical processes governing galaxy growth and evolution remains an outstanding challenge in astronomy. Constraining these processes requires observations at multiple epochs, but despite exquisite observations of galaxies in the local universe, relatively little is known about galaxies at early times. In the last decade, large photometric surveys have revealed many details about galaxies across the past 10 billion years. However, fully understanding galaxies in the early universe and how they connect to today's galaxy population requires observations of their physical properties through spectroscopy as well as photometry. Recent instrumentation advances have now paved the way for spectroscopic surveys of large samples of distant galaxies, which provide key insights into the earlier phases of galaxy evolution. In this dissertation, I use detailed photometric and spectroscopic observations and simulations to investigate the dust content, masses, and kinematic structures of star-forming galaxies at z~1.5-3, near the peak of cosmic star formation. I present results using Hubble Space Telescope (HST) grism observations of an unbiased sample of galaxies at z~1.5 from the 3D-HST survey to measure the relation between nebular and stellar dust attenuation. These constraints on the dust content of distant galaxies enable accurate measurements of star formation rates, and help to characterize the dust distribution in early galaxies. I also investigate the internal kinematics of galaxies at z~1.5-3 using moderate-resolution near-infrared spectra from the MOSDEF survey with Keck/MOSFIRE together with high spatial-resolution HST imaging. I develop a set of models to measure and interpret kinematics from spectra taken with galaxy-slit misalignments, including galaxies without spatially-resolved spectra. I then use these models to derive independent, dynamical estimates of the galaxy masses, and to constrain the amount of support from ordered versus random motions for hundreds of galaxies with Mstar ~ 10^9 - 10^11.5 Msun. Additionally, I explore the correlation of kinematic structure with other properties and constrain how the dark matter fraction in star-forming galaxies changes over time. Finally, I use mock observations of galaxies from the high-resolution MassiveFIRE cosmological simulation suite to determine how well intrinsic galaxy sizes and stellar masses are recovered from observations. I also explore the impact of random viewing angles on observed galaxy properties, which has implications for the interpretation of the scatter in galaxy scaling relations