Motivated by this initial success, the scientists tested the EAGLE simulations in more detail by comparing not just the total mass of H I, but also its distribution within galaxies to observations. The above-mentioned “Bluedisk” project has shown that this distribution is surprisingly independent of the total mass of H I as long as the galaxies’ H I discs are scaled to a common size (so called “self-similarity”). For an accurate comparison, the team now ‘observed’ the EAGLE galaxies in the same way as was done in Bluedisk. As can be seen in Fig. 3, both agree surprisingly well: EAGLE reproduces both the self-similarity between ‘normal’ and ‘H I-rich’ galaxies (red and green symbols in Fig. 3) and the detailed shape of the surface density profile – at least in the outer parts of the simulated galaxies.
Clearly visible are a number of large holes in the hydrogen disc. © MPA
Why, now, do some EAGLE galaxies contain these large holes? The scientists have not yet found a definitive answer, but it is likely that the way in which supernova explosions are modelled in the simulation plays a major role. This critical part of galaxy formation is still causing headaches for simulators: to include them in galaxy simulations in a fully self-consistent fashion, the resolution of the simulations would need to go up by many orders of magnitude. This will be, regrettably, impossible for a long time to come – even the biggest supercomputers today are just not big enough (see also highlight August 2015). As a result, EAGLE has to resort to using a highly simplified model for the effects of such supernovae. Another simplified model has to be employed for the dense interstellar matter, because a resolution level that would allow a fully self-consistent treatment can also not yet be achieved in simulations of a representative portion of the Universe. Although these simplified models produce galaxies which are realistic in many ways – such as their size – they do leave a noticeable artefact in some of the simulated hydrogen discs: the large holes discovered by the researchers.