How could the Universe form such dense and extremely hot structures only 1.4 billion years after the Big Bang? This question emerges with the recent discovery of a galaxy cluster that challenges our knowledge of cosmic evolution, presenting unexpected characteristics for its age.
Astronomers using the ALMA array in Chile have identified a cluster named SPT2349-56, observed at a time when the Universe was still young. This cluster brings together more than 30 galaxies in a compact volume, similar to the size of our Milky Way's halo. Its unusual density and heat have attracted the attention of the scientific community.
An artist's impression of the galaxy cluster in the very early Universe. It contains about 30 galaxies, three of which have supermassive black holes, and is filled with extremely hot gas.
Credit: Lingxiao Yuan. The main surprise comes from the temperature of the gas filling the cluster, called the intracluster medium. This gas is at least five times hotter than theoretical predictions, reaching levels that exceed those observed in many current clusters. Dazhi Zhou, the lead author of the study, expressed astonishment at this early heat, which indicates faster evolution than expected.
To measure this temperature, the researchers used the Sunyaev-Zeldovich effect. This phenomenon allows the detection of how photons from the cosmic microwave background, the residual glow of the Big Bang, interact with electrons in the intracluster medium. The hotter the gas, the more energy electrons transfer to the photons, leaving an observable signature in astronomical data.
Other early clusters, such as z660D or A2744z7p9OD, have been discovered at even older epochs. However, they are classified as protoclusters, as they are not yet gravitationally bound in a stable way. Unlike them, SPT2349-56 already exhibits an overheated atmosphere, showing that its evolution has been accelerated, disrupting established models.
Furthermore, star formation within this cluster is extremely intense. The member galaxies produce stars at a rate five thousand times greater than that of our Milky Way. This frenetic activity, coupled with the presence of supermassive black holes, could contribute to the unusual heat observed in the intracluster medium.
This observation opens new perspectives on the formation and heating of galaxy clusters. Astronomers are now seeking to understand the interactions between stellar formation, black holes, and the overheated atmosphere. The results have been published in the journal
Nature, offering a new look at the young Universe.
The Sunyaev-Zeldovich effect
The Sunyaev-Zeldovich effect is a key phenomenon in cosmology for studying galaxy clusters. It occurs when photons from the cosmic microwave background, the residual radiation of the Big Bang, pass through the hot gas of a cluster. By interacting with the electrons in this gas, the photons gain energy, slightly altering their frequency.
This alteration allows astronomers to indirectly measure the temperature of the intracluster medium, without needing to directly observe the gas. The effect is particularly useful for detecting distant clusters, as it does not depend on their distance, only on the density and heat of the gas.
In practice, telescopes like ALMA or observatories dedicated to the cosmic microwave background use this effect to map the distribution of clusters in the Universe. This helps understand how cosmic structures formed and evolved over time.
Applying the Sunyaev-Zeldovich effect to SPT2349-56 revealed an exceptionally high temperature, confirming that this cluster is a special case in cosmic history.