Star formation laws and gas turbulence in nearby galaxies

Star-forming galaxies continuously form new stars out of cold interstellar gas distributed in a disc. The star formation law, which tells how many stars form from a given amount of gas, is crucial to understand galaxy evolution. Since galaxy discs are seen in projection in the sky-plane, their gas and star formation rate densities are measured on a surface. These projected quantities have been used to derive a surface-based star formation law and it was found that it holds in the inner disc but fails in their outskirts. This thesis presents a new volume-based relation obtained by converting projected densities into volume densities using an accurate method to derive the gas disc thickness. This volumetric star formation law holds across the whole disc of star-forming galaxies and is likely a more fundamental relation.