The goal of my thesis is understanding the formation processes of self-gravitating, giant molecular clouds by the passage of turbulent diffuse interstellar gas through a galactic spiral arm.

In the Milky Way Galaxy, star formation occurs in cold, molecular clouds. The studies of their internal dynamics, gravitational collapses, and ensuing evolution such as formation of circumstellar disks and jets and outflows from young stellar objects form rich research fields. While these studies have significantly improved our understanding of local cloud structure and dynamical evolution, yet how local models relate to the large scale structure of giant molecular clouds is not clearly understood.

Star-forming molecular clouds are mostly found along spiral arms and it is widely recognized that the spiral gravitational potential perturbation has a strong influence on the formation and the long-term evolution of molecular clouds. Analysis of individual giant molecular clouds shows that they are internally turbulent, with velocity dispersion ten or more times greater than sound speed, gravitational potential energy in near-virial equilibrium with kinetic energy, and magnetic energy also comparable to the kinetic component. Since we are interested in the large scale structure of clouds, the galactic differential rotation is also an inevitable ingredient. In addition, the phase transition of atomic to molecular gas is thought to be very important to form gravitationally bounded objects. Therefore, modeling the formation of molecular clouds which should include all these complicated processes can be tractable only through numerical simulations.

An HST image of the Whirlpool galaxy M51 (NGC 5194), a grand design spiral (type Sc), located 37 million light years away from the Milky Way Galaxy. This image shows spiral arm structures in unprecedented detail.

The magnificent spiral arms in M51 are due to the tidal encounter with its companion, a peculiar galaxy, NGC 5195. Gaseous medium is disturbed and compressed when it passes through the arms, and turns into new young stars to shine bright. The galaxy is rotating in the counterclockwise direction.

Spiral arms are remarkably well traced by dark dust lanes and bright OB star clusters. Regularly spaced dust spurs jut out almost perpendicularly to main spiral arms. We believe that these structures form out of gravitational instabilities of gas inside spiral arms where gas surface density is high, shear is low (even reversed), and magnetic field is strong.