Astronomers Find Galaxy Groups and Clusters Have Dual Nature

A recent study led by the University of Tartu has uncovered distinctive differences, more than previously understood, between galaxy groups and clusters. The study aimed to identify two unique categories of galaxy clusters with their own unique formation and evolutionary processes. By concentrating on the cosmic web, the research was able to enhance understanding of the dynamic nature of galaxy systems and their environmental impacts.

Most astronomers have traditionally believed that the difference between galaxy groups and clusters lies in the number of galaxies they contain - with clusters containing more galaxies and groups having fewer. A team of astronomers at the University of Tartu's Tartu Observatory, led by Maret Einasto, decided to investigate further and found even more disparities between the two.

The Universe's structure can be depicted as a gigantic network, or a cosmic web, with chains of individual galaxies and small groups of galaxies connecting to massive galaxy groups and clusters containing thousands of galaxies. There are vast voids of almost no visible matter (in terms of galaxies or gases) between these systems. Superclusters, which are even larger systems, can be formed from the groups and clusters of these galaxies.

The Tartu astronomers applied data on galaxy groups, their brightest (or main) galaxies, and their surrounding environments. The aim was to integrate these data and observe whether this could provide any new insight into the potential classification of different size groups.

The study found that galaxy groups and clusters could be categorized into two classes, each exhibiting unique characteristics. The physical processes influencing the evolution and formation of the main galaxies in groups and clusters differ in richer and poorer groups. The study offered two distinct descriptions of the group's environment. They firstly described the cosmic web in terms of a general density field, with superclusters being the largest high-density regions and voids the low-density regions. Secondly, they calculated each galaxy group's distance from the nearest filament axis. This distance helps to determine whether the group is within, near, or far from the filaments.

The study used colored circles to represent galaxy groups or clusters. The richest galaxy clusters are colored red and belong to the Hercules and Leo superclusters, while the yellow, green, and blue circles symbolize the galaxy groups from the brightest to the faintest.

The main galaxies of galaxy groups were divided into those with no ongoing star formation (predominantly red galaxies) and galaxies with active star formation (young stars giving these galaxies their blue color). There were also found red galaxies with active star formation.

Upon comparing the properties of the main galaxies based on their luminosity, they discovered that groups fall into two main categories. High-luminosity groups and clusters mostly house non-star-forming red galaxies, while low-luminosity poor groups can house both non-star-forming galaxies as well the blue or red star-forming galaxies.

The differences between groups and clusters extend beyond just luminosity - each sample can be also classified based on other characteristics. Interestingly, it was revealed that the high-luminosity galaxy groups and clusters are primarily situated in high-density regions and filaments. In contrast, low-luminosity galaxy groups and individual galaxies are scattered throughout the cosmic web, including low-density areas. Paradoxically, the luminosity of poor galaxy groups within superclusters is much higher compared to those outside superclusters, despite having the same number of members.

The study also found that rich groups with main, no longer star-forming galaxies, possess differing dynamical properties than groups whose main galaxies are star-forming. Notably, in groups and clusters with non-star-forming main galaxies, these main galaxies are predominantly located in the cluster’s center. Conversely, the star-forming main galaxies can be found quite far from the group center. They also discovered that the known correlation between the stellar velocity dispersions of main galaxies and the group velocity dispersions, does not hold true in the case of very rich clusters, particularly those with non-star-forming main galaxies.

Describing the properties of the structure of the Universe and how they form and evolve is one of the fundamental tasks of cosmology. The results extend our understanding of the formation and evolution of galaxy groups and clusters and their main galaxies in the cosmic web. Rich galaxy clusters can only form in regions where the overall density of matter is sufficiently high and where there is plenty of gas necessary for star formation. In such regions, rich clusters can be joined by other (equally rich) groups and clusters. In low-density regions (the currently void areas), only rather poor groups can form, which are located quite far apart, and thus, there are few mergers.

The research results also suggest that the physical processes influencing the formation and evolution of the main galaxies in groups and clusters are different in rich and poor groups. The evolution of single galaxies and main galaxies in small groups is mainly influenced by processes in and around their dark matter haloes; the impact of other galaxies and more distant surroundings (galaxy group mergers, etc.) is important primarily in rich clusters. Our study also underlined the importance of galaxy superclusters as a unique environment for the formation and evolution of galaxies and galaxy systems.

In researching galaxies and galaxy groups, the next step of the working group will be using the new observational data, including data on very faint galaxies. Tartu Observatory participates in a number of such observation programs.

Reference: “Galaxy groups and clusters and their brightest galaxies within the cosmic web” by Maret Einasto, Jaan Einasto, Peeter Tenjes, Suvi Korhonen, Rain Kipper, Elmo Tempel, Lauri Juhan Liivamägi and Pekka Heinämäki, 22 January 2024, Astronomy & Astrophysics. DOI: 10.1051/0004-6361/202347504

Funding: Alfred P. Sloan Foundation, U.S. National Science Foundation, U.S. Department of Energy, National Aeronautics and Space Administration, the Japanese Monbukagakusho, Max Planck Society, Higher Education Funding Council for England, ICRAnet through a professorship for Jaan Einasto, Vilho, Yrjö and Kalle Väisälä Foundation, Estonian Research Council