Gas Kinematics and Dynamics: Spiral Structure and Cloud Formation in Disk Galaxies

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Star formation in disk galaxies occurs as a result of various physical mechanisms acting in the interstellar medium. Molecular clouds, the sites of star formation, grow from the diffuse interstellar medium due to a combination of gas self-gravity, galactic rotation, and magnetohydrodynamic effects. Observations have suggested that spiral arms promote star formation by compressing gas as it flows through the arms. When the density is sufficiently high, gravitational instability causes gas to collapse and form clouds. After the formation of stars within such clouds, the subsequent evolution of the cloud and the surrounding ISM is dramatically altered. Feedback effects such as stellar winds, ionizing radiation, and supernova explosions inject energy into the surrounding medium; these processes may halt the collapse, and perhaps even destroy the natal clouds. In this thesis, we study the flow of gas through the spiral arms of the grand-design galaxy M51; additionally, through numerical simulations, we model the growth of clouds in spiral arms and investigate the effect of feedback on cloud formation and disk dynamics.

We use both observational and numerical methodologies to study gas kinematics and dynamics in spiral galaxies. Using CO and H alpha velocity fields we study spiral arm streaming in M51. With numerical simulations, we investigate gravitational instability in disk galaxies, which leads to the growth of clouds. In order to study the subsequent evolution of the gaseous disk, we include feedback effects that return dense cloud gas back into the surrounding ISM.

We find that the simple description of a stationary spiral pattern in M51 is inaccurate. Our numerical models suggest that sheared features can grow regardless of the presence of grand-design spiral structure, but that spiral perturbations cause arm clouds to grow, along with distinct spiral substructure. We find that feedback can significantly affect the evolution of the gaseous disk. We suggest that the disk thickness is important in setting the rate at which stars form. The turbulence scale also needs to be considered, both for the growth of clouds and stars, as well as for the evolution of any large-scale spiral pattern.