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.
