Formation and evolution of giant molecular clouds in a barred spiral galaxy

Understanding where and how gas is converted into stars in a galaxy is important for under-standing a galaxy's formation and evolution through each epoch of the universe. Which physicalprocesses control the star formation in a galaxy is heavily debated.We are now at a stage where it is possible to investigate the giant molecular clouds (GMC)and star formation, while also taking global galactic dynamics into account. Thanks to highresolution and sensitive observations from sources such as the millimeter/submillimeter obser-vations by ALMA and infrared observations by Spitzer and Herschel, it is becoming possible tostatistically explore GMC and star forming regions through observations in nearby galaxies. Intheoretical works, we can also now investigate the formation and evolution of individual GMCsusing hydrodynamical isolated galaxy simulations with a self-consistent multiphase interstellarmedium (ISM) thanks to developments of super-computer and effective algorithms.Recent observations (high resolution, but not enough to resolve down to GMC scale yet)have shown the star formation activity changing between galactic-scale environments. The starformation efficiencies (SFEs) have systematic variations larger than one order of magnitudebetween different galaxy types and between different regions within a galaxy. This means thatthe gas density is not the only factor that determines the star formation activity in a galaxy.In particular, observations of barred galaxies showed that a central bar region has a lower SFEthan that in the spiral arm regions even when the gas surface densities are almost the same.Why does the star formation activity differ depending on the galactic structure's differentenvironments? This question is key to understanding the galactic-scale star formation and hasbeen the focus of my doctoral research. To understand this, it is important to investigate howthe formation and evolution of GMCs is affected by the galactic structures. This is becausethe GMCs are the star formation spots in a galaxy; they are formed from the cold phase of theISM, and their densest pockets are the birth place of stars.We performed three-dimensional hydrodynamical simulations of a barred spiral galaxy. Weclari ed that galactic environments and stellar feedbacks affect GMC formation and evolution,and that could explain the different star formation activities in a barred spiral galaxy. Theseworks consists of three parts. They are summarised below.