In Part I, I examine the dynamics of giant molecular clouds through simplified semianalytic models. I focus on the growth of clouds as they accrete gas. Our model clouds reproduce the scaling relations observed in both galactic and extragalactic clouds: clouds attain virial equilibrium and grow maintaining roughly constant surface densities, Σ ≃ 50–200 Msun/pc^2 and that clouds grow along the well-known linewidth-size relation. We compare our models to observations of giant molecular clouds and associated young star clusters in the Large Magellanic Cloud, finding good agreement between our models and the relationship between H ii regions, young star clusters, and giant molecular clouds.

The role of gravitational-instability driven turbulence in determining the structure and evolution of disk galaxies, and the extent to which gravity rather than feed- back can explain galaxy properties, remains an open question. To address it, in Part II I present high resolution adaptive mesh refinement simulations of Milky Way-like isolated disk galaxies, including realistic heating and cooling rates and a physically motivated prescription for star formation. The simulations resolve densities typical of the transition from atomic to molecular hydrogen, capturing the formation of gravitationally bound clouds. We present simulations both with and without stellar feedback from Type II supernova blast waves. We find gravitational instability alone can drive substantial turbulence in galactic disks and reproduce some properties of nearby star forming galaxies: Q ≳ 1, c ~ 10 km/s, without stellar feedback. Including feedback produces an ISM with a structure similar to observed disks, with the bulk of the gas in the warm or cold atomic phase, and the remainder locked up in short-lived gravitationally bound clouds. We investigate radial flows of gas and find that radial migration of gas due to gravitational instability can supply a substantial fraction of the gas needed for star formation in the inner parts of star forming galaxies.

The injection of energy and momentum into the interstellar medium by young massive stars’ intense radiation fields and their fast, radiatively driven winds can have a profound influence on their formation and environment. Massive star forming regions are rare and highly obscured, making the early moments of their formation difficult to observe. Instead, we must turn to theory to elucidate the physics involved in the formation of massive stars and massive star clusters (MSCs), which can host thousands of massive stars. In my thesis, I developed analytical and numerical techniques to study the formation of massive stars and how stellar wind feedback affects the dynamics of gas that surrounds MSCs. To estimate the initial rotation rates of massive stars at birth, I developed a protostellar angular momentum evolution model for accreting protostars to determine if magnetic torques can spin down massive stars during their formation. I found that magnetic torques are insufficient to spin down massive stars due to their short formation times and high accretion rates. Radiation pressure is likely the dominate feedback mechanism regulating massive star formation. Therefore detailed simulation of the formation of massive stars requires an accurate treatment of radiation. For this purpose, I developed a new, highly accurate radiation algorithm that properly treats the absorption of the direct radiation field from stars and the re-emission and processing by interstellar dust. With this new tool, I performed a suite of three-dimensional adaptive mesh refinement radiation-hydrodynamic simulations of the formation of massive stars from collapsing massive pre-stellar cores. I found that mass is channeled to the massive star via dense infalling filaments that are uninhibited by radiation pressure and gravitational and Rayleigh-Taylor instabilities. To determine the importance of stellar wind feedback in young MSCs, I used observations to constrain a range of kinetic energy loss channels for the hot gas produced by the shock-heating of stellar winds to explain the low X-ray luminosities observed in Hii regions. I demonstrated that the energy injected by stellar winds is not a significant contributor to stellar feedback in young MSCs.

Using 0D, 1D, and 3D models of galaxies, I explore different problems in galaxy evolution most suited to each technique. In the simplest case, a galaxy is described by a few numbers integrated via coupled ordinary differential equations. By allowing the galaxies to respond to a stochastic accretion rate, I show a natural way of generating the finite scatter observed in several galaxy scaling relations: the correlation between a galaxy's stellar mass and its star formation rate or metallicity. By comparing this simple model to observations, we constrain the process by which gas accretes onto galaxies, which must occur, but is essentially impossible to observe directly. Adding an additional dimension to the models, we explore the structure of galactic disks as a function of radius. We find that turbulence driven by gravitational instability in the disks and the resulting migration of gas can explain a wide variety of phenomena, including the age-velocity dispersion correlation of stars in the solar neighborhood, the central quenching star formation in disk galaxies, rings of star formation, and the observed radial profile of gas column densities. Finally, we run a set of fully three-dimensional galaxy simulations to try to understand what physics is responsible for basic properties of galaxies, including the rate at which they form stars, and the rate at which they eject mass in large-scale winds. We find that supernovae are capable of driving moderate metal-enhanced winds, but surprisingly they have very little effect on the star formation rates of dwarf galaxies. Instead, ordinary photoelectric heating dominates the star formation law in low-mass galaxies.