Photoionising feedback in spiral arm molecular clouds

We present simulations of a 500 pc\(^2\) region, containing gas of mass 4 \(\times\) 10\(^6\) M\(_\odot\), extracted from an entire spiral galaxy simulation, scaled up in resolution, including photoionising feedback from stars of mass > 18 M\(_\odot\). Our region is evolved for 10 Myr and shows clustered star formation along the arm generating \(\approx\) 5000 cluster sink particles \(\approx\) 5% of which contain at least one of the \(\approx\) 4000 stars of mass > 18 M\(_\odot\). Photoionisation has a noticeable effect on the gas in the region, producing ionised cavities and leading to dense features at the edge of the HII regions. Compared to the no-feedback case, photoionisation produces a larger total mass of clouds and clumps, with around twice as many such objects, which are individually smaller and more broken up. After this we see a rapid decrease in the total mass in clouds and the number of clouds. Unlike studies of isolated clouds, our simulations follow the long range effects of ionisation, with some already-dense gas becoming compressed from multiple sides by neighbouring HII regions. This causes star formation that is both accelerated and partially displaced throughout the spiral arm with up to 30% of our cluster sink particle mass forming at distances > 5 pc from sites of sink formation in the absence of feedback. At later times, the star formation rate decreases to below that of the no-feedback case.