Astronomy Colloquium

The formation and evolution of molecular gas and clouds control star formation in spiral galaxies such as the MW. I will discuss two key topics on the evolution of the gas that leads to star formation: (1) the HI and H2 phase balance and evolution during a galactic rotation, and (2) the development of dense clumps within molecular gas during a spiral arm passage. The classic/textbook scenario of ISM evolution ties the gas phase evolution and star formation, positing a rapid gas phase transition from interarm HI, to giant molecular clouds (GMCs) and star formation in a spiral arm, and then back into HI by photodissociation. This HI-H2 phase transition, however, is observed only at the outskirts of galaxies, but not in their major inner parts. Instead, the gas resides largely in GMCs even in interarm regions, but stays inactive in star formation. Therefore, the mere presence of GMCs is not sufficient for star formation, and it must be triggered in pre-existing GMCs. I will also show evidence for the development of dense molecular clumps, the precursor of star formation, predominantly in molecular gas in spiral arms.