A paper describing a set of self-consistent RHD simulations of the evolution of the Diffuse Ionised Gas (DIG) in disc galaxies was just accepted for publication in Monthly Notices of the Royal Astronomical Society. This work builds on previous models of the DIG that post-processed disc galaxy simulations from the SILCC project, and provides the first fully self-consistent model of the DIG that combines the dynamic impact of photoionisation feedback on the structure of a disc galaxy with a detailed study of the ionisation and temperature structure of the resulting DIG.
Because of the complexity of this kind of modelling, significant approximations are made: the dynamic impact of Supernova explosions (SNe) is ignored and the coupling between the radiation and the hydrodynamics is done using a pseudo-isothermal equation of state that uses constant temperatures for ionised and neutral gas (the so called two-temperature approach). As a result of these approximations (especially the lack of SNe feedback), the dynamic structure of the model DIG is not as extended as the observed DIG. However, the self-consistent treatment of the radiation field shows that the amount of gas in the ionised layer correlates with the total strength of the ionising sources. This naturally leads to a DIG in which spectral hardening is effective, which in turn explains the observed emission characteristics of the DIG.
An example of one of the simulations is shown in the movie below. Initially, the dynamics is dominated by the D-type expansion of individual HII regions surrounding the UV sources, but as sources disappear and appear in different locations, the dynamics becomes more complex. After approximately 200 Myr, a dynamic equilibrium develops between the time dependent source model and the dynamic structure of the ISM.
For a fixed time dependent model for the source positions, there is only one simulation parameter that can be explored: the ionising luminosity per source. The movie below shows the evolution of the density structure for three different values of this parameter: a low value that yields a very dense disc that is predominantly neutral, an intermediate value that has a more extended ionised layer and less neutral gas, and a high value that has no clear neutral disc.
By combining the density and the ionisation structure of the cells in the simulations and integrating out along the y axis of the simulation box, mock emission maps can be made for HI (neutral gas) and Halpha (ionised gas). These are shown in the movie below.