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Investigating the mechanisms that shape galaxies in and around massive clusters

Periodic Reporting for period 1 - ClusterGal (Investigating the mechanisms that shape galaxies in and around massive clusters)

Reporting period: 2017-10-01 to 2019-09-30

Astronomical observations have revealed that galaxies in clusters — dense associations of hundreds or even thousands of galaxies — are markedly different from their cousins living in a more isolated environment, such as the Milky Way. The colour of cluster galaxies is typically red, rather than blue, they have often ceased forming new stars several billion years ago, and their morphology is mostly round or elliptical, rather than dominated by a thin disk with spiral arms. These differences imply that, somehow, galaxies "know" about the large-scale environment in which they live: their formation and evolution must be markedly different in clusters than in the more typical, less crowded regions of the Universe. However, astronomers have so far only had a sketchy picture of exactly how galaxies interact with their environment, and how this shapes the galaxies that we observe in clusters.

Uncovering these interaction mechanisms is an important fundamental science question, for a number of reasons. Our picture of galaxy formation is necessarily incomplete without an understanding of how it works in the extreme environment of galaxy clusters. Furthermore, galaxy groups harbour as much as one third of all galaxies in the Universe, and the processes governing their evolution are thought to be essentially a milder variant of those operating in clusters. Finally, galaxy clusters have emerged as promising “tools” to study fundamental questions of astronomy and cosmology, such as the nature of dark energy and dark matter. To reach the required precision, these measurements require an accurate understanding of the cluster galaxies and their special evolution.

The ClusterGal project was therefore set up to shed light on cluster galaxies through a new high-resolution cosmological hydrodynamical simulation suite, Hydrangea. These simulations are based on the successful EAGLE project and specifically target massive galaxy clusters and their large-scale filamentary surroundings at the highest resolution currently achievable on such scales. With this setup, we could investigate the formation and evolution of cluster galaxies in a realistic way. This was done by comparing the simulations to state-of-the-art observations, extracting galaxy histories from the simulations, and comparing to the predictions of simple, intuitive models.
During the project, a new code, “Spiderweb” was developed that can robustly trace galaxies over time as they orbit in a cluster, even when they experience mergers with other galaxies, lose material due to stripping by gravitational or hydrodynamical forces, or experience close encounters with other galaxies. A second code developed during the project allows a precise assignment of matter belonging to galaxies orbiting in a massive cluster. In combination, they allow a robust and quantitative determination of how galaxies are stripped and re-shaped in a cluster.

The simulation predictions have been compared to state-of-the-art observations in a number of aspects, including the distribution of galaxy masses across a cluster (the stellar mass function) and its variation across cosmic time. This has revealed encouraging agreement with the observations, confirming that the simulations model the environmental impact on stars in a galaxy in a realistic way. The simulated fraction of cluster galaxies that are no longer forming stars (“quenched”), however, turns out to be artificially high: in the simulations star formation is interrupted too aggressively for cluster galaxies. Further work to understand the cause of this discrepancy is ongoing.

Despite these detailed shortcomings in the simulations, ClusterGal has already led to a number of important new insights into the evolution of cluster galaxies. These include the finding that galaxies in clusters are surprisingly robust to tidal disruption, that the most massive galaxies in clusters are built up hierarchically from successive mergers of smaller galaxies, and that cluster galaxies with surprisingly high black hole masses are mostly relics of more massive galaxies that have been tidally stripped of most of their stars.

Motivated in part by the shortcomings discovered in the Hydrangea simulations, the new EAGLE-XL simulation project is now underway to produce a new generation of simulations that overcome these problems. The researcher is a key member of this project, and has led efforts to improve the modelling of supermassive black holes, which have a significant impact on the structure of galaxy clusters and the galaxies within them.

The results have been disseminated to the scientific community through so far 10 articles in peer-reviewed journals, and presentations by the researcher at three international conferences, six topical workshops, and visits to three large astronomy departments. In addition, the project has been featured in a special space focus edition of the EU CORDIS magazine, making the results also accessible to the general European public.
The ClusterGal project is a first attempt of studying the evolution of cluster galaxies with the aid of a realistic, self-consistent, cosmological hydrodynamical simulation. Previous studies were limited to using either simplified ‘semi-analytic’ models, which need to make explicit assumptions about the impact of the cluster environment (or ignore the presence of ordinary, baryonic matter completely), or simulations that did not even predict realistic isolated galaxies. The project therefore represents a significant step beyond the previous state of the art.

The simulation data, including the catalogues produced during this project, will shortly be made publicly available, so that the entire scientific community can benefit from the work invested in them.

The researcher has already obtained additional funding from the Dutch Science Organisation (NWO) through a VENI grant to extend this research programme. Over the next three years, we will therefore exploit the foundation laid by the ClusterGal project further, by determining the origin of morphological transformations of cluster galaxies, investigating how stripping of gas leads to the appearance of spectacular `jellyfish’ galaxies, and analysing the impact on galaxies of the filamentary network linking clusters.
Synthetic image of a massive galaxy cluster from the Hydrangea simulations and its galaxies