The Cosmic Dispersion Measure in the EAGLE Simulations
Abstract
The dispersion measure (DM) of fast radio bursts (FRBs) provides a unique way to probe ionised baryons in the intergalactic medium (IGM). Cosmological models with different parameters lead to different DM-redshift (DM-z) relations. Additionally, the over/under-dense regions in the IGM and the circumgalactic medium of intervening galaxies lead to scatter around the mean DM-z relations. We have used the Evolution and Assembly of GaLaxies and their Environments (EAGLE) simulations to measure the mean DM-z relation and the scatter around it using over one billion lines-of-sight between redshifts 0<z<3. We investigated two techniques to estimate line-of-sight DM: `pixel scrambling' and `box transformations'. We find that using box transformations (a technique from the literature) causes strong correlations due to repeated replication of structure. Comparing a linear and non-linear model, we find that the non-linear model with cosmological parameters, provides a better fit to the DM-z relation. The differences between these models are the most significant at low redshifts (z<0.5). The scatter around the DM-z relation is highly asymmetric, especially at low redshift (z<0.5), and becomes more Gaussian as redshift approaches z3, the limit of this study. The increase in Gaussianity with redshift is indicative of the large scale structures that is better probed with longer lines-of-sight. The minimum simulation size suitable for investigations into the scatter around the DM-z relation is 100~comoving~Mpc. The DM-z relation measured in EAGLE is available with an easy-to-use python interface in the open-source FRB redshift estimation package FRUITBAT.