If 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 stubborn 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 magazine 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 from the cosmic dawn
In the contemporary celestial bestiary, two large families of galaxies are distinguished. On the one hand, the spirals, veritable stellar nurseries with rotating arms full of gas, tirelessly perpetuate the creation of stars. The Andromeda Galaxy (M31), our closest galactic neighbor, the Whirlpool Galaxy (M51) and the Sombrero Galaxy (M104) are part of this category.
On the other, 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 previously struggled to explain how the latter acquired their characteristic shape.
The dominant hypothesis suggested a gradual transformation from rotating disks to a three-dimensional configuration. However, the new observations from this study 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 galaxies located 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 decisive for understanding early galactic architecture.
Interstellar dust, a true marker of galactic activity, plays a fundamental role in this study. Its presence directly testifies to the existence of gas, an essential raw material for the birth of stars. The data collected thus demonstrate a very compact spatial distribution of this dust, a phenomenon which enters in direct contradiction to classic 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 precisely reconstructing the three-dimensional geometry of dusty regions. This allows researchers to work with images in excellent resolution.
The data collected proves that these primitive galaxies already presented a pronounced spheroidal morphologyvery 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, calling into question the paradigm of a progressive transformation of the shape of galaxies over time. In reality, the mechanisms of matter concentration and intensive star formation operated much more quickly and efficiently than theoretical models predicted until now.
Interpretation of observations by numerical simulation
The cosmological simulations carried out by the team shed additional light on 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 nuclei. These central regions, true stellar crucibles, catalyze 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.
Exploration prospects therefore promise to be particularly fruitful with the recent deployments of a new generation of observatories. The JWST-Euclid space tandem will provide a unique look at 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 making it possible 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 elucidate the action of interstellar gas and reveal the mechanisms that govern the feeding of star formation regions and sculpt the shape of contemporary galaxies.
Although we cannot predict with certainty the exact pace of these future discoveriesit is undeniable that the next decade promises to be particularly exciting for astrophysics. Looking forward to the sequel !
- Elliptical galaxies took their spherical shape from the earliest ages of the Universe
- The compactness of interstellar dust proves that elliptical galaxies formed much earlier than expected.
- These new observations challenge classic models and will certainly be supplemented by other discoveries in the coming years.
