Star Formation in Gravitationally Unstable Disk Galaxies: From Clouds to Disks

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