2021MNRAS.505.5356B

The dispersion measure (DM) of fast radio bursts (FRBs) provides a unique way to probe ionized baryons in the intergalactic medium (IGM). Cosmological models with different parameters lead to different DM-redshift (DM-z) relations. Additionally, the over/underdense 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 1 billion lines of sight at 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 a non-linear model, we find that the non-linear model with a dependence on 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 Gaussiana as redshift approaches z = 3, the limit of this study. The increase in Gaussianity with redshift is indicative of the large-scale structure that is better sampled with longer lines of sight. 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.