We finally know more about the formation of giant galaxies

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While spiral galaxies, like our Milky Way, are gradually revealing their secrets, their elliptical cousins ​​have until now remained a source of questions for the scientific community. Fortunately, an international team of astrophysicists has just lifted the veil on one of the most persistent mysteries of our cosmos: the process of formation of these imposing galaxies.

The results of their observations were published on December 4 in the prestigious journal Nature. Observations from data collected by the ALMA (Atacama Large Millimeter/submillimeter Array) observatory, one of the most powerful instruments in the world for observing our Universe.

Spherical giants shaped since the cosmic dawn

In the contemporary celestial bestiary, two large families of galaxies stand out. On the one hand, spirals, veritable stellar nurseries with swirling arms filled with gas, tirelessly perpetuate the creation of stars. The Andromeda Galaxy (M31), our closest galactic neighbor, the Whirlpool Galaxy (M51) or the Sombrero Galaxy (M104) are part of this category.

On the other hand, the ellipticals, massive spheroidal structures, shelter venerable stellar populations, formed more than 10 billion years ago. M87 (Virgo A), NGC 4552 or M32 are from this family. Cosmological models have struggled until now to explain how these latter had acquired their characteristic shape.

The dominant hypothesis suggested a progressive transformation of rotating disks towards a three-dimensional configuration. However, the new observations of this study call into question this scenario.

A dazzling birth in the primordial Universe

The observations carried out by the team of researchers focused on a particularly representative sample: more than a hundred galaxieslocated in a precise time window, between 2.2 and 5.9 billion years after the Big Bang. This period, characterized by intense star formation activity, is crucial for understanding primitive galactic architecture.

Interstellar dust, a true marker of galactic activity, plays a fundamental role in this study. Its presence directly attests to the existence of gas, the raw material essential for the birth of stars. The data collected thus demonstrate a very compact spatial distribution of this dust, a phenomenon that directly contradicts classical models predicting a disk-shaped distribution.

To reach these conclusions, the team developed a new technique for analyzing interferometric data (sets of measurements combined from multiple antennas to form high-definition images of the cold Universe) from ALMA. This methodological approach makes it possible to overcome the limitations of traditionally used observations by accurately reconstructing the three-dimensional geometry of the dusty regions. This allows researchers to work with images in excellent resolution.

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The data collected prove that these primitive galaxies already exhibited a pronounced spheroidal morphology, very different from the flattened structure characteristic of galactic disks. This early geometric configuration establishes a direct link with the elliptical galaxies that we observe in our contemporary cosmic neighborhood.

This morphological similarity suggests a previously unknown evolutionary continuity, challenging the paradigm of a progressive transformation of the shape of galaxies over time. In reality, the mechanisms of matter concentration and intensive star formation have operated much more quickly and efficiently than theoretical models had predicted until now.

Interpretation of observations by numerical simulation

The cosmological simulations carried out by the team provide additional insight into the physical mechanisms at work. The combined action of cold intergalactic gas flows and interactions between galaxies would lead to the concentration of gas and dust in compact cores. These central regions, true stellar crucibles, catalyze a particularly intense star formation.

It is thanks to the analysis of the ALMA archives that this scientific breakthrough was able to take place, which proves that the collaborative model is very effective in modern astronomy. The pooling of observational data, collected over several years, allows international teams of researchers to maximize the scientific potential of the instruments.

The prospects for exploration are therefore particularly fruitfulwith the recent deployments of a new generation of observatories. The JWST-Euclid space tandem will provide a new perspective on the distribution of stellar populations within the precursors of elliptical galaxies. The Extremely Large Telescope, with its monumental 39-meter mirror, will achieve a spatial resolution that will allow us to dissect the physical processes at the heart of these ancient galactic structures. The synergy between ALMA and the Very Large Telescope, for its part, will help to elucidate the action of interstellar gas and to reveal the mechanisms that govern the feeding of star-forming regions and sculpt the shape of contemporary galaxies.

Although we cannot predict with certainty the exact pace of these future discoveries, it is undeniable that the next decade promises to be particularly exciting for astrophysics. Looking forward to the next one !

• Elliptical galaxies took their spherical shape in the early ages of the Universe
• The compactness of interstellar dust proves that elliptical galaxies formed much earlier than expected.
• These new observations challenge classical models and will certainly be supplemented by other discoveries in the coming years.

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