Maret Einasto

A high percentage of galaxies in low-density regions are not star-forming

At the largest scales, the structure of the Universe resembles a huge network, which is called the cosmic web. In the cosmic web, nodes are formed by rich galaxy clusters, which are connected by chains of galaxies filaments. Between filaments there are voids, which contain almost no visible matter. Voids are of various size and some of them may be really huge. The largest systems in the cosmic web are galaxy superclusters, which have the richest galaxy clusters. Superclusters occupy only 1% of the total volume of the Universe. The rest of the volume of the Universe is covered by underdense regions penetrated by filaments of poor galaxy groups and single galaxies: for example galaxies that do not have bright galaxies in their neighbourhood. However, such galaxies may have dwarf satellite galaxies. There are no rich galaxy clusters in underdense regions.

The formation of galaxies started already in the very early Universe. At present, according to the average age of stellar populations, galaxies can be divided into those in which star formation ended already a long time ago (so-called quenched galaxies) and those that actively form stars even now. It is well known that the central parts of rich galaxy clusters are mostly populated by galaxies that fell into clusters billions of years ago. In such galaxies, star formation ended several billions of years ago under the influence of various processes in the cluster environment. This means that stellar populations in such galaxies are old. Typically, such galaxies are elliptical and red; the red colour comes from the colour of the old stars in them.

Studies of galaxy clusters and groups have shown that in the regions more densely populated by galaxies there are always more galaxies with old stellar populations. Furthermore, it has been shown that as the density of galaxies decreases, there are fewer old galaxies, and in the lowest density environments there are almost no old galaxies; instead, such environments are populated by galaxies that are blue and either mostly spiral or irregularly shaped. Their blue colour suggests that stars actively form within them. This trend is also well explained by theories that describe processes that end star formation in galaxies in dense environments.

In our recent paper, which has been accepted for publication in the journal Astronomy and Astrophysics, we asked the following question: is it possible that such old galaxies which populate central regions of galaxy clusters can also be found in big voids between superclusters, which are populated by poor groups and single galaxies, and where the evolution of galaxies should be considerably different from that in the cluster centres? Quite surprisingly, our study showed that almost one third of galaxies in voids are very similar in their properties to those galaxies that populate rich clusters. The formation of stars ended at least four billion years ago, and their stellar masses are almost as large as the stellar masses of galaxies in clusters. In groups and clusters the properties of galaxies are very similar. The largest differences between galaxy properties are within groups and clusters, where the brightest cluster or group galaxies have considerably older stellar populations and higher stellar masses than the satellite galaxies around them. 

The distribution of galaxy groups in the sky plane. Red circles denote groups and clusters in superclusters, and other colours show groups in underdense regions. 




Therefore, our study has shown that although the global environment strongly affects the formation of galaxy groups and clusters, its influence on galaxy properties is surprisingly small. Our results suggest that galaxy properties are mostly affected by their birthplace in the cosmic web in two aspects. Firstly, the birthplace determines whether a galaxy becomes a member of a rich galaxy cluster (perhaps even the brightest galaxy!), becomes a member of a poor group, or stays single. Secondly, galaxy properties are determined by their close environment, the dark matter halo around them, and processes at work within the halo. The influence of the larger environment is smaller than thought earlier.

This result may considerably affect our understanding of galaxy evolution. To understand galaxy evolution better, we plan to analyse galaxies in larger volumes and include fainter galaxies than in this study.

Read the article here.


Maret Einasto, Rain Kipper,  Peeter Tenjes, Jaan Einasto, Elmo Tempel, Lauri Juhan Liivamägi
Tartu Observatory,
Department of galaxy physics and cosmology

Einasto et a. 2022, Death at watersheds: galaxy quenching in low-density Environments. A&A 668, A69


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