Abstract

It is thought that the diffuse ionised gas (DIG) is ionised via radiation leaking from HII regions, however photoionisation models struggle to replicate observed line ratio trends without introducing unconventional secondary sources of ionisation. I will present a new analysis of the DIG in star-forming galaxies with a high-resolution, isolated Milky Way-like simulation. I will show how the synthetic observations are able to match observed line ratio trends, and that these trends arise from an increasing temperature and hardening radiation field in the DIG . I will demonstrate that this is due to early stellar feedback clearing HII regions within ~5 Myr, unveiling a luminous population of stars with intrinsically hardened SEDs which ionise low-density gas on kiloparsec scales. The state of the DIG in star-forming galaxies is driven by recent star formation, not a secondary source of ionisation.