The interstellar medium (ISM) in galaxies is heated and ionised by shock waves caused by stellar winds of massive stars and by supernova explosions. Elements formed in stars and during the explosion are expelled into the ISM, and shocks compress and change the cold medium, so new star formation occurs. In addition, particles are accelerated in the shocks to relativistic energies and thus form the population of non-thermal particles, which we can observe as Galactic cosmic rays. In recent years, multi-wavelength studies of structures in the ISM have also shown that collisions of giant cold gas clouds can lead to the formation of numerous highly massive stars and to additional heating of the interstellar plasma due to compression.

In nearby galaxies, the large effective area of Athena together with the large field of view of WFI and the high spatial resolution of X-IFU will allow us to study both different structures in the ISM like HII regions, superbubbles, and supernova remnants and the global properties of the hot interstellar plasma, which has only been possible in the closest galaxies like the Magellanic Clouds so far. In addition, we will be able to study galactic outflows and galaxy collisions and their impact of star formation and galaxy evolution. For these observations exposure times of only a few 10 ks will be necessary for galaxies up to ~20 Mpc to obtain high-statistics images and spectra with WFI, allowing a homogeneous study of a large sample of nearby star-forming galaxies. The figure shows a simulated Athena/WFI image of only the diffuse X-ray emission in the Whirlpool galaxy (M51) located at 8 Mpc distance for an exposure time of 10 ks. The difference between the emission in the nuclear region of the main galaxy, the disk, and the merging galaxy is clearly seen.