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. The second part of this dissertation focuses on the origin of turbulent motions of cold gas in galactic discs. Since turbulence is expected to be quickly dissipated by the gas viscosity, it needs to be maintained by some energy source. This latter has remained unknown so far, as even the prime candidates, supernova explosions of massive stars, have been considered insufficient. A crucial improvement is obtained in this thesis by realising that the thickness of galactic gas discs slows down the dissipation and reduces the energy required to sustain turbulence. This brings to the conclusion that supernova explosions can feed the turbulence of the cold gas, finally solving the conundrum.