Wednesday, July 22, 2009 - 3:15pm
Rahul Shetty (CFA):
"Gaseous Spiral Arm Structure and Cloud Formation in Disk Galaxies"
Abstract. The formation of giant molecular clouds (GMCs) and larger HI "superclouds" is a crucial step towards star formation. Using global 2-dimensional magneto-hydrodynamic simulations, we study the growth of clouds in the warm diffuse ISM resulting from gravitational instability, turbulence, and the gaseous response to a stellar spiral potential. Gravitational instability causes sheared features to grow regardless of the presence of a background spiral perturbation. However, a sufficiently strong spiral potential is required to produce true "arm spurs," consisting of inter-arm structures emerging from clouds that grow in the main spiral arms. We implement feedback in the clouds to return the dense gas back into the diffuse phase. Feedback raises the ambient turbulence levels in the disk, truncating star formation in a given cloud, but resulting in the subsequent growth of clouds in the interfaces of colliding shells. The net effect of feedback, however, is to reduce the fraction of dense gas and thus the overall star formation rate. I will discuss the Kennicutt-Schmidt star formation index of our simulations in the context of expectations and observations. Finally, I will also briefly discuss the effects of projection, in both observed 2D column density maps and 3D position-position-velocity cubes, in assessing the structure of gaseous clouds.