The formation and life-cycle of giant molecular clouds is a crucial part of understanding both the evolution of galaxies and the varying environments in which the star formation process occurs. The origins of the physical phenomena that govern giant molecular clouds and the star formation within them include external effects caused by the galactic environment (such as galactic potentials and external radiation fields) as well as more local effects such as stellar feedback and turbulence. Recently, we performed simulations of a galactic spiral arm consisting of complexes of giant molecular clouds that included ionising radiation from young stars. Unlike previous work in this area, which typically simulated isolated clouds, our models include a number of clouds along a spiral arm, and thus ionisation fronts propagate in a much more realistic environment. The initial conditions were achieved by extracting a 500 pc² section of spiral arm from a galaxy scale simulation by Dobbs & Pringle (2013) and increasing the resolution.

Using the simulations we examine the effect of photoionising feedback caused by the most massive stars forming within the clouds. Lyman continuum photons emitted from these massive stars ionise the surrounding gas creating bubbles – HII regions – which are bounded by expanding shocks. Photoionisation has the highest energy budget of the early onset stellar feedback mechanisms. Comparison of simulations with and without photoionisation shows that the timing and location at which stars form is significantly affected (Fig. 1). While the final fraction of gas converted to stars remains relatively unchanged, the time taken is reduced by a factor of nearly 2. This is due to star formation in nearby dense gas being compressed by shocks from the surrounding ionised regions. The result is an acceleration of the star formation rate and a much wider distribution of star formation across the region.