The FABLE simulations of galaxies, groups and clusters

The FABLE simulations of galaxies, groups and clusters

Clusters of galaxies collapse from the densest regions of the early Universe to become the largest objects in existence by the present day. Their unparalleled sizes make them powerful tools to probe the growth of large-scale structure and further our understanding of cosmology. At the same time, clusters represent unique astrophysical playgrounds in which galaxies interact with each other and with the intracluster gas, which itself is churned up by the powerful energy output of supermassive black holes. This process, known as AGN feedback, stems from black holes at the centres of galaxies swallowing gas, converting it into outflows of energy and impacting not only their galaxy, but also the surrounding intracluster medium millions of light-years away.

The inclusion of AGN feedback in cosmological hydrodynamical simulations of galaxy formation has proven to be a key ingredient for producing realistic galaxies in a representative region of the universe. Projects such as the Illustris simulation have met with great success in this area and thus have made great strides in our understanding of galaxy formation and evolution, as well as guiding observations. Yet whilst this model matches a wide range of observed galaxy properties, it struggles to reproduce other important properties of galaxy clusters and groups such as the contents of the intracluster medium.

Figure 1: The main panel shows the gas distribution of one of the largest fable clusters, revealing an array of interconnecting filaments and infalling galaxies and groups. Panel 1 shows an observed spiral galaxy with an enormous tail of gas shining in X-rays and panel 2 shows a similar view for one of our simulated galaxies. This gas has been stripped from the galaxy as it falls into the dense cluster environment. Panel 3 highlights a smaller group of galaxies of various shapes and sizes, while panel 4 shows the structure of AREPO’s moving hydrodynamical mesh through a slice of the central galaxy.

With the FABLE project we have built upon the success of Illustris by improving the models for feedback from AGN and stars so that we can simultaneously produce realistic galaxies, groups and clusters, thereby completing our picture of the Universe across this vast range of scales. Consisting of high-resolution simulations of more than thirty groups and clusters performed with the state-of-the-art moving-mesh code AREPO, FABLE reproduces a number of important observables such as the build-up of stars in galaxies and the thermodynamic properties of the intracluster medium.

FABLE further reproduces a range of integrated cluster properties and their interdependencies as represented via the oft-studied cluster scaling relations. These relations are vital for the cosmological interpretation of observed cluster data, the availability of which is set to be greatly expanded by numerous ongoing and future surveys, for example with observations of the Sunyaev-Zel’dovich effect via SPT-3G and ACTpol or in X-rays with the eROSITA and Athena observatories. Yet as larger cluster surveys beat down the statistical uncertainties, we are increasingly limited by our incomplete understanding of cluster physics and its impact on the cluster scaling relations, which are required to link cluster observables to their fundamental masses.

Figure 2: The evolution of the fable cluster shown in the first figure, starting from just 2.2 billion years after the Big Bang (left) through to the onset of a major merger between two smaller clusters 5.7 billion years later (right) that will eventually form one of the largest possible gravitationally-bound objects in the simulated (and real) universe.

Cosmological hydrodynamical simulations are in a unique position to quantify these effects and thereby facilitate the use of clusters as precise cosmological probes. Indeed, careful comparison of our results with observed scaling relations hints towards a non-negligible bias in observational studies that attempt to measure the masses of clusters from their X-ray emission under simplifying assumptions about the clusters’ dynamical state.

Furthermore, the increased size and depth of future surveys means that many new clusters will lie at vast distances from which their detectable emission may have taken many billions of years to reach us. This means it is vital to understand how the cluster scaling relations change over time so that they can be applied reliably to the most distant objects in upcoming datasets. While current observations are severely limited in their ability to constrain such evolution, simulations allow us to study the scaling relations for a well-defined sample of objects over the whole history of the Universe. Upcoming results from FABLE demonstrate a significant departure of the evolution of the relations from the simple model typically assumed in observational studies, which has important implications for interpreting the incoming wealth of new cluster data.