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Abstract

Recent observations have shown that star formation is correlated with the molecular phase of the interstellar medium. Molecular gas tends to organize itself into large and roughly self-gravitating entities called Giant Molecular Clouds (GMCs). These objects should, therefore, play a key role in controlling star formation and defining its modes. However, their physical properties, formation and evolution mechanisms are still poorly understood - especially in spiral galaxies. The new PdBI Arcsecond Whirlpool Survey (PAWS) offers, for the first time, the possibility to study the molecular gas distribution in a grand-design spiral galaxy dominated by dynamical phenomena. The aim of this thesis is to evaluate the importance of galactic environments for the gas organization. Via a thorough analysis of the gas kinematics I study the structure of M51’s gravitational potential and spiral arm streaming motions. This analysis provides several insights on the differing nature of the molecular and atomic phase of the interstellar medium. I also find evidence for a kinematic m = 3 mode that explains the asymmetry of the spiral arms. To investigate the effect of the dynamical environment on the molecular gas I have generated the largest extragalactic GMC catalog to date using an automatic algorithm that accounts for the observational biases. Differences in the cloud properties suggest that environments, and in particular dynamical effects, strongly influence the organization of the gas in spiral galaxies and provide a way to discriminate between the various mechanisms of cloud formation and evolution that have been proposed in the literature.