  A field with particularly exciting results over the past few years is the
study of the interaction of cosmic rays with interstellar matter. For star
formation to take place, gas and dust need to be sufficiently cold for gravity
to overcome thermal pressure, and the ionisation fraction must be low enough to
enable substantial decoupling between the gas and the Galactic magnetic field.
As soon as the visual extinction is of the order of 3-4 magnitudes, the
ultraviolet photon flux from the interstellar radiation field is fully
quenched, thus the only source of ionisation and heating is provided by
low-energy cosmic rays. We will briefly focus on the Galactic and local origin
of cosmic rays and on their effects on medium ionisation.

Astrophysics of Galaxies