Discovery of a new superstructure of distant galaxies: the underside of a 10-year study

What is a discovery in astrophysics? The first preliminary results, or the final confirmation? All this at once?

Practical case: we have just discovered a new structure of distant galaxies with telescopic observations in Hawaii and Arizona.

This collection of young galaxies has been observed since the universe was only 3 billion years old. Our calculations show that it is destined to develop into a very massive superstructure. More surprisingly, this structure of galaxies calls into question our models, since the observed star formation rates were not predicted at this level …

This almost fairy-tale-like scientific study began more than ten years ago with observations from space.

The history of the universe in two sections

The main lines of the history of the universe seem to be globally understood by the scientists who have developed the “cosmological concordance model” (or simplified Big Bang model), which is consistent with very many observations and has a strong prediction; no matter how many questions there are, such as the nature (and the very existence) of dark matter or dark energy, the physical mechanisms that work in the first moments, the origin of the very first stars and galaxies, the properties of gravity or even the persistence of certain voltages between measurements that do not completely agree. Thus, new, more accurate observations and further theoretical developments are needed to advance these issues.

In this frame of mind supported by accurate measurements, the universe has expanded for 13.8 billion years as it cools, hydrogen was synthesized during the first three minutes, light was decoupled from ordinary matter (called baryonic) about 370,000 years ago by produce fossil light, the first stars and galaxies probably appeared before 400 million years ago, and since then large structures have formed and evolved to today give rise to complex galaxies that are mainly located in huge galaxy clusters, which are themselves located in supercluster of galaxies.

A370 galaxy cluster.
NASA, ESA / Hubble, HST Frontier Fields

Among the many ways in which one can better trace the history of the structuring of the universe, by following the evolution of galaxies from the very distant universe by observing them, is a trail that has been explored for a long time, always rich and fruitful in results. Recently, a rather dense and emerging field of study has taken shape, namely to try to understand how galaxies are not formed and evolve, but here galaxy clusters, which have become the largest structures in the universe.

cluster of galaxies

Galaxy cluster is a collection of thousands of galaxies, hot gas and … 80% dark matter (this material that gravitates, but can not be observed, currently hypothetical, but whose presence is consistent in astrophysics), weighing about 1014 total solar masses. These are fascinating structures to understand as they can tell us about the values ​​of certain cosmological parameters (such as the density of dark matter in the universe) as well as many physical processes such as gravitational collapse (the fact that matter is attracted to a denser area of ​​gravity attraction) in the most massive halos, the connection between dark and baryonic matter, large-scale energy exchanges, or the evolution of galaxies in a dense medium.

However, the “parents” of these clusters (ancestors) are almost unobserved in the past because they are very difficult to detect, whereas one would like to be able to trace their relationship, their past and thus compare the data with the predictions of models and simulations. From then on, teams of astrophysicists doubled their efforts and tricks to observe and try to identify these elusive clusters of distant galaxies, which are called protohobes of galaxies.

Why are protocols of galaxies difficult to detect? Unless you have a spectroscopic examination that gives exact distances and velocities of the galaxies, or use a method that detects hot gas (in X-rays or in millimeters), the images of the sky alone, although accurate, do not allow to distinguish the galaxies that belongs to a cluster or protohob from the galaxies in the foreground or background: the images show many galaxies, without it being easy to determine exactly whether there is a protohob or – if there is – what are the member galaxies.

What do we think we know and what do we know about protocols of galaxies?

The protocols therefore constitute the missing pieces in the puzzle, which makes it possible to accurately reconstruct the formation of the large structures of the universe.

Example of an evolutionary model from a galaxy protocol cluster (right) to a galaxy cluster (left) (Fig. 13 from Shimakawa et al., 2018).
Shimakawa et al., 2018

Since the formation models for nearby galaxy clusters appear to be quite accurate compared to available data, the scientific community uses them logically to predict what galaxy protohobes may look like in the early universe. It is therefore believed to know that protoclusters of galaxies can be formed quite early (before the first billion years) and already house the most massive galaxies of their generation. However, several protocol clusters will eventually form a cluster of galaxies by successive mergers. Therefore, there is not a single progenitor protocol of a cluster of galaxies, but many that would make their detection even more difficult. We also believe that protocluster galaxies are undergoing a very intense and rather time-consuming phase of star formation. At the same time, large reservoirs of cold baryonic gas had to be present in these intergalactic protoclusters to feed this very persistent star formation.

a protocol of galaxies discovered in 2016.
Wang et al., 2016, A&A

Several protocols of galaxies have already been discovered and confirmed as such. One of the most distant is when the universe was about 800 million years old and a few dozen correspond at times between 1.2 and 3.3 billion years. Most contain galaxies that are already very massive and “dry” (no longer forming stars), not quite in line with predictions. A team in 2018 developed a cluster formation scenario based on 3 galaxy protohobes. Although this model is relevant, it seems that the small number of protoclusters and the lack of diversity currently limit our understanding.

The discovery of galaxy protocols G237

It is in this context that “our” protocols of galaxies that respond to the sweet abbreviated name of G237 (pictured below) bring something new. First, it is confirmed by beautiful observations made in Hawaii and Arizona to be at a time when the universe was 3 billion years old (redshift z = 2.16). It is an impressive task to arrive at this confirmation with several dozen galaxies scanned, led by my dear colleagues Dr. Yusei Koyama (Japan and Hawaii) and Dr. Mari Polletta (Italy and France).

Area of ​​the sky showing the proto-cluster of galaxies G237 when the universe was about 3 billion years old.
ESA / Herschel and XMM-Newton; NASA / Spitzer; NAOJ / Subaru; ESO / VISTA; Polletta et al., Koyama et al.

This cosmic epoch corresponds to the maximum density of star formation in the universe. Then, previous space observations allow us to measure the rate of star formation. Surprise: the speed is extreme, several thousand solar masses a year: galaxies form so many stars that our models and simulations struggle to explain this hectic pace. Finally, by predicting the mass it will achieve when G237 becomes a “true” large galaxy cluster, we calculated that it would weigh 5 or 6 times 1014 solar masses, or the equivalent of the supercluster of galaxies to which our galaxy, the Milky Way, belongs. So much information in a seemingly standard protocol!

What have we actually discovered?

The two scientific articles we have just published in 2020 and 2021 on this discovery relate to the confirmation that G237 is in fact a protocluster of galaxies. But how did we choose this place to point out the largest telescopes in the world? Not by chance, of course.

the candidate protocols of galaxies discovered by Planck and then Herschel.
ESA and Planck Collaboration / H. Dole, D. Guéry & G. Hurier, IAS / University Paris-Sud / CNRS / CNES

The story begins in 2009, with the launch of the European Planck and Herschel space missions. Together with Planck, we had discovered about 2,000 galaxy protocluster candidates scattered across the sky, and together with Herschel and Spitzer (from NASA), I myself led the program with early, more comprehensive studies of about 200 of these candidates in infrared. Their special properties: they should form impressive amounts of stars, given their intense flux in infrared. So in 2015, if we had no confirmation, we had very strong reasoned suspicions about the nature of protoclusters for the majority of these candidates. But it was still at the hypothetical stage.

Numerous observations from space in infrared and on Earth with telescopes in several wavelengths, several doctoral dissertations, several student internships, a lot of questions, sometimes setbacks, proposals for telescopes and ‘ANR submitted (and rarely accepted), failed attempts, discussions and analyzes later , here we are with this first confirmation in hand, with the key to a surprise in terms of the disagreement between observations and models.

Discovery is therefore a long process that started long before the launch of Planck and Herschel, as there had been traceability forecasts upstream for one or more decades. It is not finished, because it is necessary to resolve this disagreement, to get other confirmations, by preparing the arrival of exquisite data, which will come from the space missions Webb (launched in December 2021) and Euclid (launched in 2023, ESA -mission). A breathtaking adventure!

Hundreds of candidate protocol clusters of galaxies observed by Herschel.
ESA / Herschel / SPIRE / Planck Consortia and H. Dole, D. Guéry, IAS, CNRS, Paris-Saclay University

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