Galaxy clusters are the largest known gravitationally bound structures in the Universe and contain up to thousands of galaxies. We currently have at least a broad-brush understanding of how local massive clusters affect their members, both via tidal and hydrodynamic processes. However, our knowledge is much more limited at higher redshift (z > 1-2), when structures are younger and still in formation and gas densities and star formation rates were much higher. Studies of the highest redshift progenitors of local clusters — called proto-clusters — are currently very sparse and limited.

Up to date, we have not had systematic comparisons among structures over a range in redshift, mass, and evolutionary stage and models of proto-clusters are largely untested. This lack of a systematic characterization of protoclusters prevents us from obtaining a global view of how the environment affects the properties of galaxies at the earliest stages.

We have assembled a Team of scientists with highly complementary expertise in clusters and protoclusters and with diverse research specialisations (observations and simulations, integrated and spatially resolved data, different wavelengths and focusing on different cosmic epochs) to tackle questions that cannot be addressed in isolation and combine efforts towards understanding cosmic cluster evolution. This team has access to complementary datasets and simulations that are ideally suited for evolutionary studies.


The immediate goals of the proposed research are:

  • Investigate the nature of the galaxies within (proto)clusters, by combining protocluster samples already in hand and taking into account the systematics and biases of the different selections. We will draw a coherent picture of cosmic cluster evolution, aided by detailed comparisons with simulations.
  • Characterize the role of protoclusters in the evolution of galaxies destined to live in low-z clusters, by homogeneously combining existing samples of galaxies in (proto)clusters and the co-eval field.
  • Determine the fuel of SF in high-z (proto)cluster and field galaxies by measuring the contents and characteristics of different gas phases and by comparing those results to theoretical predictions.
  • Quantify the relative importance of different physical processes in quenching galaxies in different environments, by spatially resolving tracers of stellar mass, gas, and SF within galaxies.
  • Prepare to exploit the future wealth of public and proprietary protocluster data, by characterising the diversity in the protocluster population, and quantify the impact of selection bias on their properties.



Summary of the activities:

Thanks to the support from ISSI, we gathered a team of scientists based across the world involved in the detection and characterisation of clusters and protoclusters, both from the theoretical and observational point of view. Understanding the role of the environment in shaping galaxy properties at different epochs is still one of the most important open questions in galaxy evolution, as observations of large samples of cosmic structures, needed to perform statistical analysis, have been proven to be extremely challenging. 

The team met twice in Bern in 2022 and has been meeting online approximately once per week since then. 

During the first meeting we assembled the most comprehensive list of spectroscopically confirmed protoclusters at 2<z<3.5. That catalog is currently used to select the most suitable targets for multiple proposals requesting time to state-of-the-art telescopes across the world, aimed mainly at understanding how gas flows in and out from galaxies located in different environmental conditions and what is its effect on galaxy properties. We are putting great effort in collecting data for our analysis. 

We have also identified candidate z > 1.3 protoclusters and clusters in the LSST Deep Drilling Fields, through the characterization of galaxy overdensities in a Spitzer/IRAC colour-selected samples using criteria that were optimized for protocluster purity using realistic simulations.

Simultaneously, we have been addressing the role of environments at intermediate redshift following two other lines of research. One one side, we are exploiting theoretical modeling, both semi-anlaytic models and hydrodynamical simulations, to make predictions on how massive environments quench star formation in galaxies. On the other side, we are exploiting publicly available cluster samples at z~1-2.5 to characterize the stellar mass function and its evolution in the densest regions of the universe. 

Two publications came out of the ISSI Team, two are currently under revision and three are in the advanced stage of preparation. The collaboration is still active and we foresee other publications in the next few years.






Last edit: March, 2024