The intergalactic medium (IGM) is the rarefied material that spans the vast distances between galaxies in the Universe. In our DiRAC thematic project, we model the ionisation and thermal structure of the IGM using a combination of hydrodynamical structure formation simulations and GPU accelerated radiative transfer. The results form part of the “Sherwood” and “Sherwood-Relics” simulation projects.
The image shows the temperature of the IGM predicted by one of the Sherwood-Relics models, performed with the DiRAC Memory Intensive and Data Intensive Cambridge facilities (credit: E. Puchwein). It shows a region of the Universe 24 million light years across, at a time when the Universe was only 6 per cent of its current age. This corresponds to period known as the epoch of reionisation, when the first stars and galaxies were forming and emitting the ultraviolet (UV) radiation that ionised and heated the IGM. The colour scale shows IGM temperatures ranging from cold (dark blue at a few hundred Kelvin), to very hot (red at >100,000 Kelvin). The sharp boundaries between the dark blue and yellow regions mark the transition from gas that is ionised and heated by the UV radiation emitted by the first stars, to cold, neutral gas that has yet to be illuminated.
Selected science highlights for 2021include new measurements of the IGM temperature at redshifts z~2-4 from the Ly-a forest. The results are consistent with the epoch of helium reionisation occurring rapidly at z~3 (Gaikwad et al. 2021, MNRAS, 506, 4389). We have also completed the first simulations of the 21-cm forest in late reionisation models, showcasing the potential of SKA1-low observations to place informative lower limits on the soft X-ray background and neutral hydrogen spin temperature at z~6 with 21-cm absorbers in the diffuse IGM (Šoltinský et al. 2021, MNRAS, 506, 5818). Finally, we have recently presented a new measurement of the mean free path for the ionising photons through the IGM at z~6 (Becker et al. 2021, MNRAS, 508, 1853). This has yielded a surprisingly low value of ~3.6 cMpc/h, which is a factor of ~2 smaller than expected even for a very late end to reionisation at z~5.3. This suggests that the ionising photon production from galaxies at z~6 may have to be dramatically boosted to ensure reionisation ends by z~5.3.