Photoionizing feedback in spiral arm molecular clouds

We present simulations of a 500 pc(2) region, containing gas of mass 4 x 10(6)M(circle dot), extracted from an entire spiral galaxy simulation, scaled up in resolution, including photoionizing feedback from stars of mass >18M(circle dot). Our region is evolved for 10 Myr and shows clustered star formation along the arm generating approximate to 5000 cluster sink particles approximate to 5 per cent of which contain at least one of the approximate to 4000 stars of mass >18M(circle dot). Photoionization has a noticeable effect on the gas in the region, producing ionized cavities and leading to dense features at the edge of the H II regions. Compared to the no-feedback case, photoionization 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 ionization, 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 per cent 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.