The interplay of gas and stars in nearby galaxies

To understand the evolution and appearance of galaxies it is crucial to study the interplay between stars and the interstellar gas and dust. I will present results and plans to study the ISM and star formation in the nearby galaxies M51 and M33. For M51, we combined maps of the molecular and atomic gas to derive the total gas surface density. Taking into account the velocity dispersions of the molecular and atomic gas, and the stellar surface densities, we then derived the total Toomre Q parameter of the disk, describing its stability against gravitational collapse. A map of Q shows values around 1 on the spiral arms, indicating self-regulation at play. In the second part of my talk I will present our plans to study M33. Here, we will combine ground based CO and HI data with Far-Infrared data taken with the Herschel Space Observatory in the framework of the open time key project HERMES.