The universe hosts a zoo of galaxies with different sizes and shapes. Our Galaxy, the Milky Way, is a rather large and massive spiral galaxy, with 250 billion stars and about one new star forming every year. At early epochs of the universe, galaxies were smaller but forming stars at higher rates. How actively stars are forming in galaxies is an important aspect of galaxy evolution as it determines the chemical make-up and the dynamical state of galaxies. Understanding the mechanisms that regulate the star-formation activity of galaxies and how those mechanisms vary with the evolutionary stage of a galaxy are active areas of research and necessary steps to address fundamental questions such as: When and where do stars form in galaxies? What is fueling star formation? How does the star-formation activity shape galaxies through cosmic times?
There are multiple size scales in the process of star formation. On the largest, galaxy-wide scales, the material in galaxies - the interstellar medium (99 percent gas and 1 percent dust) - needs to cool. It then contracts and fragments into clouds down to smaller scales until the collapse of clouds on the smallest scales. While the small-scale processes are best studied in our own Milky Way, the current main challenge to understand star formation in external galaxies is to quantify the amount, composition, and state of the material on the large scales.
To this end, the goal of my project was to better characterize the properties of the interstellar medium of galaxies that is linked to the formation of new stars. My project relied on an innovative approach, combining the analysis of a large set of new multi-wavelength observations from space and ground-based telescopes and building state-of-the-art object-specific spectral synthesis models. With this, I have constrained, for the first time, the cooling, physical conditions, topology, masses, and star-formation efficiencies in a range of galaxies, which are key parameters that link the interstellar medium to the star-formation activity in galaxies.
